Spinach Lasagna

INGREDIENTS

• 1/4 cup olive oil
• 1 small yellow onion, finely chopped
• 2 teaspoons coarse salt
• 2 pounds baby spinach
• 4 tablespoons unsalted butter
• 1/4 cup all purpose flour
• 1 quart milk
• 1 packed cup grated Parmesan (3 ounces)
• 1/4 teaspoon freshly grated nutmeg
• 1/4 teaspoon ground black pepper
• 2 medium zucchini (about 1 pound), sliced lengthwise into 1/4-inch-thick planks
• 8 ounces no-boil lasagna noodles
• 6 ounces mozzarella, shredded
• 1/3 packed cup (1 ounce) grated Pecorino Romano