Broiled Tuna Stuffed Tomatoes

My mother made this one with their homegrown tomatoes when I visited this weekend, and it turned out fabulously!  She served it with a salad and sourdough bread.

1/2 onion, chopped
2 TBSP butter
5 oz mushrooms, chopped
2 cloves garlic, minced
1 (6.4 oz) can tuna
3 cups fresh bread (sourdough or french), chopped
1.5 cup shredded mozzarella
6 tomatoes

Saute onion in butter.  When softened add mushrooms and cook until tender.  Add garlic and cook one more minute.  Move to a mixing bowl and add tuna breadcrumbs and half of cheese.  Half tomatoes and scoop out innards to form bowls.  Fill with tuna mixture, drizzle with oil, and top with remaining mozzarella.  Bake for 12 minutes at 350.  Turn on broiler just until tops are lightly browned.