Wring the spinach dry in a clean kitchen towel. Using your fingers, loosen the leaves.
Bring a large pot of water to boil for the pasta. Place a rack in the upper third of the oven and preheat the broiler.
In a skillet, heat the EVOO, one turn of the pan, over medium. Melt the butter into the oil and when it foams, add the onion and garlic. Season with salt and pepper and cook, stirring, 3 to 5 minutes to soften. Stir in the flour and cook another minute. Whisk in the milk and cook until thickened, 5 to 7 minutes. Season with nutmeg and reduce heat to low.
Salt the boiling water and cook the pasta to al dente. Reserve 1/2 cup of the cooking water and add to the sauce to thin a bit. Add the spinach to the pasta pot, stir, then drain and toss with the sauce; season. Arrange in a casserole dish and dot with the ricotta. Top with the mozzarella and pecorino-romano and broil until browned, 6 to 8 minutes.