Preheat the oven: Set your oven to 425°F (220°C). A hot oven is the key to browning.
Prep the mushrooms: Wipe mushrooms with a damp paper towel to remove dirt. Trim tough ends.
If the mushrooms are small, leave them whole; if large, halve or quarter them so pieces are similar in size.
Toss with oil and seasonings: On a large sheet pan, combine mushrooms with olive oil, garlic, salt, pepper, and thyme. Toss to coat. Spread them in a single layer with space between pieces.
Crowding leads to steaming instead of roasting.
Roast the first round: Roast for 15 minutes, stirring once halfway through so they brown evenly and release moisture.
Add Parmesan: Pull the pan out, sprinkle the mushrooms with Parmesan, and toss gently. Return to the oven for another 8–10 minutes, until the cheese melts, crisps at the edges, and the mushrooms are deeply browned.
Finish with lemon and herbs: While still hot, toss with lemon juice and parsley. If using, add a small pat of butter and let it melt into everything.
Taste and adjust: Add a pinch more salt, a crack of pepper, or another squeeze of lemon if needed.
Serve right away.