Clean mushrooms and remove stems. Chop stems and set aside.
In a large skillet, melt butter over medium heat. Add onion and garlic and cook until softened. Add chopped mushroom stems and cook for an additional 2 minutes.
Stir in flour and cook for 1 minute. Gradually add milk and stir until sauce thickens. Add Worcestershire sauce, thyme, salt, and pepper. Stir in shrimp and crabmeat.
Fill each mushroom cap with seafood mixture. Sprinkle Parmesan cheese on top.
Bake in preheated oven for 15 minutes, or until cheese is melted and mushrooms are tender.
Interesting Facts
Mushrooms are a low-calorie and low-fat food.
Shrimp and crab are both excellent sources of protein.
Stuffed mushrooms can be served as an appetizer, side dish, or main course.