Stuffed Pasta Shells with Ricotta, Spinach, and Mushrooms

Ingredients

• ounce 340-gram box jumbo shells
• tablespoon olive oil
• small yellow onion
• garlic cloves
• ounces sliced shiitake mushrooms
• teaspoon fine sea salt
• ounces baby spinach
• pound fresh ricotta cheese
• 2 ounces grated pecorino romano cheese, divided
• teaspoon freshly ground black pepper
• egg
• ounces low-moisture mozzarella, shredded
• marinara sauce