Here’s a simple but exceptionally tasty way to cook portobello aka flat mushrooms (also called field mushrooms or “BBQ mushrooms”). The appeal lies in how the herb butter soaks deeply into the mushrooms, and in the sliced green onions that lend a distinct flavour and attractive emerald colour to the dark, roasted mushrooms.

This recipe is ideal for serving mushrooms as the centrepiece of a meatless meal, or as a starter for a multi-course dinner!

Roasted large mushrooms

Everybody loves mushrooms, and this low-effort recipe for serving chunky portobellos makes a great vegetable side to accompany any number of dishes, like sizzling steaks, Sticky Grilled Chicken and Garlic Prawns. With their beefy texture and topped with this flavoursome melted butter, they’re also substantial and interesting enough to star as a main course for vegetarians or omnivores alike.

Herbs bring fragrance and colour, while the green onions add another layer of interest to the flavour and shifts things a step away from the usual garlic-and-butter routine.

How good does this look!!! (That’s a rhetorical question, by the way… )

Overhead photo of Roasted Large Mushrooms with Green Onion Thyme Butter on a tray, ready to be served

What you need for these roasted Flat Mushrooms

Here’s all you need to make these roasted Flat Mushrooms:

Ingredients for Roasted Large Mushrooms with Green Onion Thyme Butter

Large mushrooms – Any large mushrooms that are around 10cm /4″ or larger in diameter are ideal for this recipe. You just need mushrooms large enough so you can slather the cups with the green onion and herb butter. And remember, they’ll shrink a fair bit when cooked!

You’ll find them sold labelled with various names here in Australia. Flat mushrooms, field mushrooms, BBQ mushrooms, portobello mushrooms. Just go by size!
Butter – Softened, so we can mix in the herbs.
Thyme and parsley – The herbs of choice here. Thyme, because thyme loves mushrooms, and parsley for freshness.
Green onions / shallots – Called scallions in the US, this adds a vibrant splash of green as well as fresh allium goodness. The onions stand in as a refreshing change from delicious-but-a-little-predictable garlic. (Of course, feel free to also add garlic – it certainly won’t hurt!)

Mushrooms – Cleaning, preparing, storing

How to clean mushrooms – Clean mushrooms by removing dirt using a cloth or paper towel. If you’re going all out or need to get into the cracks and crevices, use a pastry brush! Though these days, grocery store mushrooms are sold pretty clean.

It’s best not to wash with water because mushrooms are extremely porous and so if they absorb water, it can make them slimy or mushy.

Should I remove mushroom gills? Mushroom gills are totally edible and I never remove them, unless I’m making specific types of stuffed mushrooms where I want to make room for more filling. Some people say they don’t like the flavour of the gills (eg dusty flavour), I’ve never experienced that.

However, if you don’t like the gills, just remove them using a spoon. Cradle mushroom in one hand, gently work the tip of a spoon under the gills and scoop around – it will come off with little effort.

The stem – If your mushroom has a very long stem, trim it so it’s the height of the edge of the mushroom, or snap it off (with fresh mushrooms, the stem will break off neatly).

How to store mushrooms – Always store mushrooms in a breathable bag, such as a brown paper bag or wrapped in a tea towel. If you use plastic bags or containers, the mushrooms will sweat, the surface will go slimy and the shelf life of the mushrooms will be considerably shorter.

How to make Roasted Flat Mushrooms

It could not be quicker or easier – 3 easy steps!

Mix softened butter with green onions and herbs;
Slather onto mushrooms; and
Roast 25 minutes until cooked through. That’s it!

Green onion thyme butter to put on mushrooms to be roasted

Fork picking up Roasted Large Mushrooms with Green Onion Thyme Butter

Satisfying and yet so simple. We need more recipes like this in the world!

So, what do you think? How will you serve these – as a main or as a side? – Nagi x

Roasted Flat Mushrooms with Green Onion, Thyme & Butter

This can be made with any large mushrooms. Roasting them underside-up allows the herb and green onion butter to seep deep in between the mushrooms gills.

Covering with foil helps the mushrooms stay nice and moist, while helping the herbs and green onions retain their vibrant emerald colour. It looks so fresh and stunning against the deep brown of the mushroom gills!

Simple, easy and exceptionally delicious.

Serve as a vegetarian main for 3 – 4 people, starter or side for 6 to 8, or as part of a meatless meal spread.

Course Side Dish

Cuisine Western

Keyword BBQ mushrooms, field mushroom recipe, flat mushroom recipe

Prep Time 10 minutes

Cook Time 30 minutes

Servings – 8

Calories 230cal

Author Nagi

Ingredients

8 large mushrooms (flat, portobello, “BBQ”), around 10 – 12cm / 4 – 4.5″ wide, stem tips trimmed (Note 1)

Herb & Green Onion Butter:

100g / 7 tbsp unsalted butter , softened
1 cup green onion (US:scallions), thinly sliced
1/3 cup parsley , finely chopped
1 tbsp fresh thyme leaves (sub 1 tsp dried)
3/4 tsp salt
1/4 tsp black pepper

Garnishes (optional):

3 tbsp chopped scallions
1 dozen thyme stalks

Cups – Metric

Instructions

Clean mushrooms: See Note 1.
Preheat oven to 200°C / 390°F (180°C fan).
Mix butter: Mix the Herb & Green Onion Butter ingredients together in a medium bowl.
Smear mushrooms: Place mushrooms gills side up on a large tray. Smear the butter mix over the mushroom surfaces. Cover with foil (preserves green colour of herbs).
Bake: Bake 25 minutes, until mushrooms are tender.
Serve! Arrange on a platter, including all butter and juices on tray. Scatter with thyme stalks and extra scallions over the mushrooms (for a fresh touch), and serve.

Notes

1. Mushrooms – Use any type of large mushrooms. If they are much smaller than specified, use more. The idea here is just to use large mushrooms big enough to hold the butter!

It’s best not to wash with water because mushrooms are extremely porous and so if they absorb water, it can make them slimy or mushy.

The stem – If your mushroom has a very long stem, trim it so it’s the height of the edge of the mushroom, or snap it off (with fresh mushrooms, the stem will break off neatly).