Creamy Spinach and Mushroom Lasagna

INGREDIENTS

• 1 tablespoon unsalted butter, at room temperature, for greasing the baking dish
• 1 teaspoon kosher salt
• 12 ounces cremini mushrooms, cleaned and quartered
• 2 cloves garlic, minced
• 3 tablespoons extra-virgin olive oil
• 2 cups shredded mozzarella
• 1/8 teaspoon kosher salt
• 4 large egg yolks, at room temperature
• Extra-virgin olive oil, for drizzling
• Two 5-ounce bags baby spinach, coarsely chopped
• 1/2 teaspoon freshly ground black pepper
• 1 recipe Fresh Pasta, recipe follows, or nine 8- by 5-inch fresh lasagna noodles
• 1 medium onion, chopped
• 1 1/2 cups shredded mozzarella
• 1 3/4 cups whole milk, at room temperature
• 1/4 packed cup chopped fresh basil
• 2 cups heavy cream, at room temperature
• 1/3 cup all-purpose flour
• 2 cups grated Pecorino Romano
• 2 cups cake flour
• All-purpose flour, for dusting
• 1 cup all-purpose flour, plus extra for dusting
• 1/4 cup extra-virgin olive oil