To make the sauce: In a 10" to 12" sauté pan over medium heat, melt the butter and add the leeks and 1/4 teaspoon of the salt. Cook for about 5 minutes, until softened and translucent. Stir in the flour. Cook for about 1 minute, then stir in the milk. Once the milk begins to boil, whisk in the cheeses. Cook the mixture, stirring frequently, until it's the consistency of thick gravy. Stir in the remaining salt, pepper, and nutmeg. Set aside., To make the filling: Preheat the oven to 375°F. Remove the sausage from its casing and brown it in a 12" skillet. Drain and set aside. Toss the diced butternut squash with the sugar, 2 tablespoons olive oil, and the 1/2 teaspoon salt. Spread the squash on a baking sheet and roast on the middle rack of your oven for about 40 minutes, checking it periodically. The squash is ready when it turns golden brown and is easily smashed with a fork. Leave the oven on, you'll be using it again., Remove the large stems from the turnip greens. Bring a large pot of heavily salted water up to a boil and set up an ice bath nearby. Blanch the turnip greens for about 30 seconds, then shock them in the ice bath. Once the greens are completely cool, drain them well and wrap them in a clean dish towel to wring out additional water. Roughly chop the greens., Butter a 9" x 13" baking dish. Spoon in the greens, cooled squash, sausage, and cubed fontina. Stir to combine. Pour the sauce on top. Bake on the middle rack of the oven for 15 minutes., To make the topping: Toss the cubed bread with the Pecorino Romano, olive oil, and salt, and spread it on top of the casserole. Bake an additional 15 minutes, or until the bread is toasted and the sides of the casserole are bubbly., Remove the casserole from the oven, and serve it hot or warm. Refrigerate any leftovers for up to several days.