From https://www.thebakerchick.com/white-spinach-lasagna/
	(adapted from serious eats)

(edited for time, spices 2021-01-18)

 Yield: 10-12 servings
Prep Time: 2 hours  (how long it took us, original time 40 minutes)
Cook Time: 60 minutes
Total Time: 3 hrs

Ingredients

    For the Spinach:
    4 tablespoons butter
    1 small onion, minced (about 1/2 cup)
    6 medium cloves garlic, minced
    Kosher salt and freshly ground black pepper
    2 lbs of frozen spinach, thawed and drained

    For the Ricotta:
    2 pounds fresh ricotta cheese
    2 eggs
    4 ounces finely grated Parmigiano-Reggiano cheese
    1 teaspoon freshly grated nutmeg
    1/4 teaspoon cayenne pepper (or more or less if you want a kick or not)
    For the White Sauce:
    5 tablespoons butter
    1/4 cup plus 1 tablespoon flour
    5 cups whole milk
    12 ounces grated low moisture mozzarella cheese
    12 ounces grated gruyere cheese

    For Assembly:
    16-20 no-boil lasagna noodles


Instructions
For the Spinach/Ricotta Mixture:

    In a large saucepan (I like using my big 5 qt dutch oven just to have a lot of room.) melt the 4 tablespoons of butter. Add the garlic and onions and sauté until golden. Add the spinach and stir for a few minutes to marry all the flavors. Turn off heat and set aside.
    In the bowl of a food processor fitted with a steel blade, add half the ricotta, the eggs, parmesan cheese, nutmeg and salt & pepper. Add optional cayenne. Process for a few minutes until blended and smooth. Transfer to a large bowl. (don't wipe the food processor bowl out.)
    To the food processor add the spinach/garlic mixture and pulse until finely chopped. Add to the bowl with the ricotta mixture as well as the rest of the ricotta cheese. Stir until blended and smooth.

For the White Sauce:

    In a large saucepan (I like to wipe out the dutch oven a bit and use that again,) melt the 5 tablespoons of butter and then add the flour- whisking constantly until golden. Add the milk gradually, about a cup at a time, stirring well between each addition. When thick and bubbly, add 3/4 of the mozzarella and 3/4 of the gruyere. Add salt and pepper to taste.

To assemble:

    In a 9x13 lasagna pan, start by spreading a small amount of the white sauce onto the bottom of the pan. Top with a layer of noodles (I used 4 across for each layer,) Spread 1/3 of the spinach mixture on top of the noodles, followed by 1/4 of the sauce. Repeat 3 more times, noodles- spinach- sauce. Topping with a 4th layer of noodles, the last 1/4 of sauce and the remaining cheese.
    Cover tightly with foil, and bake at 400F for 30 minutes. Remove the foil and bake for another 30 minutes or until the cheese is golden and bubbly, and internal temperature registers 165.