In a microwave, defrost the spinach, then wring dry in a clean kitchen towel. Bring a large pot of water to a boil, salt it, add the pasta and cook until al dente. Drain and return the noodles to the pot.
While the pasta is working, in a medium skillet, heat the EVOO, 1 turn of the pan, over medium heat. Add the onion and garlic and cook until tender, 8 to 10 minutes. Add the spinach, breaking it apart, to heat through. Season with salt and pepper. Remove the pan from the heat. Position an oven rack in the center of the oven and preheat the broiler. While the onion cooks, in a medium saucepan, melt the butter over medium heat. Whisk in the flour for 1 minute, then whisk in the milk; season with salt, pepper and nutmeg. Cook until thickened, about 5 minutes. In a bowl, whisk a little of the sauce into the egg yolks, then whisk the egg yolk mixture into the sauce in the pan and turn the heat to the lowest setting. In a bowl, mix together the panko, gruyere and parmigiano-reggiano.
Stir the spinach and sauce into the noodles to combine, then transfer to a casserole dish. Top with the panko-cheese mixture. Broil in the middle of the oven until bubbly and brown, 3 to 5 minutes.