Spinach and Cheese–Stuffed Chicken Breasts

Ingredients

• 2 thick, boneless, skinless chicken breasts
• 6 tablespoons (3/4 stick) unsalted butter, divided
• 1/2 onion, finely chopped
• 1-1/2 cups finely chopped mushrooms
• 3/4 pound fresh baby spinach
• 1 clove garlic, minced
• salt
• freshly ground black pepper
• 3/4 cup ricotta cheese
• 1/4 cup grated Parmesan cheese
• 1-1/2 teaspoons dried basil
• 1 cup crumbled feta cheese
• 1 large egg, lightly beaten
• 1 cup Italian-style bread crumbs