Cook the noodles – Bring a large pot of salted water to a rolling boil and cook the lasagna noodles according to package directions. Drain, toss with olive oil, and drape over a colander to cool.

Make the sausage gravy – Heat 3 tablespoons of olive oil in a large skillet over medium-high heat. Add sausage and scallion whites, breaking sausage into bite-sized pieces. Cook until browned (about 8 minutes). Sprinkle with flour, stir to absorb, then gradually add 2 cups milk. Once thickened, add the remaining milk, along with salt, pepper, and nutmeg. Simmer until the sauce coats the back of a spoon, then remove from heat.

Combine cheeses – In a medium bowl, mix mozzarella, Cheddar, and Parmesan until well blended.

Assemble the lasagna – Grease a 9×13-inch baking dish. Spread a thin layer of sauce on the bottom. Add 4 noodles, followed by 1/3 of the sauce, half of the spinach, 1/3 of the scallion greens, and 1/3 of the cheese mixture. Repeat the layers with the remaining noodles, sauce, spinach, scallions, and cheese, finishing with a top layer of sauce and cheese.

Refrigerate – Cover with plastic wrap and refrigerate overnight for deeper flavor and better texture.

Bake – Preheat oven to 350°F. Remove plastic, cover with foil, and bake for 45 minutes. Remove the foil for the last 10–15 minutes to achieve a golden, bubbly top.

Finish & serve – Top with remaining scallion greens, parsley, and a sprinkle of black pepper. Let rest 15 minutes before slicing.