Heat your oven to 375ºF and transfer the thawed spinach into a mesh sieve. Press well to remove as much liquid as possible and set aside.
Place a 10- or 12-inch cast iron skillet over medium heat. Add in the butter and onion, season with salt and pepper, and cook for 3 to 4 minutes or until tender. Add in the garlic and continue to cook for 1 minute. Scatter over the flour, stir to combine, and cook for an additional minute to toast.
Slowly whisk in the warm milk, turn the heat down to medium-low, and simmer for 5 minutes or until thickened. Season with salt, pepper, a pinch of nutmeg, and the crushed red pepper flakes and transfer the sauce to a spouted measuring cup or a medium bowl. Turn the heat off under than pan and set aside.
Add the spinach to a large bowl. Drain and finely chop the artichokes and add to the bowl along with the ricotta, Parmigiano Reggiano cheese, egg, and pesto. Season with salt and pepper as well as a pinch of red pepper flakes and mix well to combine.
Assemble the lasagna by spreading a quarter of the cream sauce across the bottom of the cast iron skillet. Lay a third of the noodles in a single layer on top, breaking off pieces of the noodles as needed. Top with half of the ricotta mixture and another quarter of the cream sauce. Lay over another third of the noodles on top followed by the remaining ricotta mixture and another quarter of the sauce. Top with the remaining noodles, spread over the rest of the cream sauce, and scatter over the mozzarella or fontina.
Spray the dull side of a piece of aluminum foil with non-stick cooking spray. Cover the lasagna, dull side down, and bake for 45 minutes. Carefully remove the foil and continue to bake for 15 to 20 minutes or until golden brown on top.
Allow the lasagna to sit for at least 10 minutes before slicing and serving with more Parmigiano Reggiano.
