Pat mushrooms dry and slice evenly.
Heat olive oil in a large skillet over medium heat until shimmering.
Add garlic and shallots (if using); sauté 1–2 minutes until fragrant.
Increase heat to medium-high; add mushrooms in a single layer. Let them sear undisturbed for 2–3 minutes.
Stir in salt, pepper, and thyme; cook for 5–7 minutes until mushrooms are browned and tender.
Optional: Stir in lemon juice, red pepper flakes, or other desired toppings in the last minute.
Remove from heat; garnish with fresh parsley. Serve warm.