Place oven rack in middle of oven and preheat broiler or oven to 425 degrees F.
Defrost the spinach in the microwave for 10 minutes on high.
While spinach defrosts, heat a medium sauce pot over medium heat. Cut butter into small pieces and melt. Whisk in flour and cook a minute or 2 then whisk in milk. Bring sauce to a bubble, as it begins to thicken, drop heat to low and season it with nutmeg, salt and pepper, to your taste.
Combine ricotta cheese, a little salt and pepper, lemon zest and garlic in a bowl.
Cook pasta to al dente, 5 to 6 minutes in salted water.
Remove spinach from the microwave and wring it completely dry in a kitchen towel. Open the towel and separate the spinach with your fingers. Pile up the sliced artichokes on your cutting board.
Drain pasta well. Ladle just enough white sauce in the bottom of a lasagna dish to lightly coat it. Top with 1/3 of the pasta. Top the pasta with a layer of half of the ricotta cheese dotted with half of the spinach and artichokes. Thinly cover the veggies with more white sauce then top liberally with grated cheese. Repeat these layers then top your lasagna with the remaining 1/3 of the cooked ravioli, the last of the white sauce and some grated cheese.
Place under hot broiler or into a preheated 425 degree F oven until sauce browns and bubbles, 3 minutes