Preheat the oven to 375 degrees F (190 degrees C). Brush a large baking sheet with about 1/4 cup melted butter.

Arrange mushroom caps, gill sides up, in a single layer on baking sheet.

Combine lobster, 1 cup mozzarella cheese, crushed croutons, remaining 1/2 cup melted butter, crabmeat, and garlic. Spoon into mushroom caps.

Bake in the preheated oven until lightly browned on top, 10 to 12 minutes. Sprinkle with remaining 1/4 cup mozzarella cheese; serve hot.