Meat-Free Stuffed Shells

INGREDIENTS

• 1 (12 ounce) box jumbo pasta shells
• 2 tablespoons butter
• 1 cup sliced mushrooms
• 1/2 onion, chopped
• 2 cloves garlic, minced
• 1 (12 ounce) package firm tofu
• 1/2 cup dry bread crumbs
• 1/2 (10 ounce) package frozen chopped spinach, thawed and drained
• 1/4 cup grated Parmesan cheese
• 1 teaspoon Italian seasoning
• 1 egg (optional)
• 2 cups shredded mozzarella cheese
• 1 (16 ounce) jar spaghetti sauce, or as needed