Prepare the mushrooms: Clean mushrooms with a damp cloth or soft brush. Slice evenly; smaller mushrooms can be left whole for texture variation.
Heat the pan: In a large skillet, heat olive oil or butter over medium-high heat.
Sauté aromatics: Add garlic and onion/shallots (if using) and cook until fragrant, about 1–2 minutes.
Cook the mushrooms: Add mushrooms to the pan in a single layer (work in batches if necessary). Cook, stirring occasionally, until they are golden brown and tender, about 7–10 minutes.
Season: Add salt, pepper, and thyme. Stir to combine.
Optional finishing touch: Drizzle with balsamic vinegar or stir in heavy cream for extra richness. Cook 1–2 more minutes until heated through.
Serve: Transfer to a serving dish and garnish with fresh parsley. Serve hot as a side, main, or topping.