Spinach-Stuffed Sole

1 tsp olive oil
1/2 lb fresh mushrooms, sliced
1/2 lb fresh spinach, chopped
1/4 tsp oregano leaves, crushed
1 clove garlic, minced
11/2 lb sole fillets or other white fish
2 Tbsp sherry
4 oz (1 C) part-skim mozzarella cheese, grated
as needed nonstick cooking spray