Balsamic Bean Casserole with Stuffed Mushrooms

For Stuffed Mushrooms: 1 package mushrooms, stems removed 1/4 yellow onion, finely chopped 1 clove garlic, minced 1/2 can diced tomatoes 1 tablespoon balsamic vinegar 1/2 teaspoon each basil and oregano For Casserole: 1 can Great Northern beans (undrained) 1 can black beans (undrained) 1 1/2 cans diced tomatoes with roasted garlic 2 cloves garlic, chopped 3/4 yellow onion, chopped 2 cups rice 1-2 cups balsamic vinegar 1 teaspoon each basil and oregano Salt and pepper to taste Vegetable stock if needed