Heat oil and butter in a large skillet set over high heat.
Add mushrooms in an even layer; cook, without stirring, until mushrooms release their water and it evaporates.
Once browned, stir mushrooms, and season with salt and pepper.
Add shallots and garlic; cook until fragrant.
Add thyme and sherry; cook until liquid has almost evaporated. Season with salt and pepper.
Stir in cream if desired; cook until heated through, about 1 minute.