Cheesy Spinach Lasagna

Name: Cheesy Spinach Lasagna
Ingredients: For the Pasta; 450 grams ‘00’ pasta flour or all-purpose flour, plus more for rolling out; 50 grams semola rimacinata or semolina flour; 285 grams eggs, from about 5 large eggs and 2 large egg yolks; Or, if you're not making your own pasta:; 1 pound store-bought lasagna noodles (not the no-boil kind); For the Lasagna Filling & Assembly; 1 1/2 pounds (680 grams) spinach or a mix of hearty greens like chard, kale, dandelion, and mustard greens; 1/4 cup (55 grams) unsalted butter or extra-virgin olive oil; 1/2 yellow onion or 1 shallot, minced; 4 garlic cloves, thinly sliced; Kosher salt and freshly ground black pepper; 1 (15- or 16-ounce) container (454 grams) whole-milk ricotta; 4 ounces (113 grams) mascarpone; 2 1/2 ounces (75 grams) finely grated Parmigiano Reggiano, plus more for topping; Freshly grated nutmeg; 1 large egg, beaten; 1/4 cup (55 grams) unsalted butter; 1/3 cup (55 grams) all-purpose flour; 1 quart (975 ml) whole milk; 6 ounces (170 grams; 1½ cups) grated Fontina, plus more for topping; 4 ounces (113 grams; 1 cup) grated low-moisture mozzarella, plus more for topping