White Spinach Lasagna

For the Spinach:

butter

small onion, minced (about ½ cup)

medium cloves garlic, minced

Kosher salt and freshly ground black pepper

lbs of frozen spinach, thawed and drained

For the Ricotta:

fresh ricotta cheese

eggs

finely grated Parmigiano-Reggiano cheese

freshly grated nutmeg

cayenne pepper (or more or less if you want a kick or not)

For the White Sauce:

butter

plus 1 tablespoon flour

whole milk

grated low moisture mozzarella cheese

grated gruyere cheese

For Assembly:

no-bake lasagna noodles