Creamy Cheddar Pasta with Scallion Bread Crumbs

Heat oven to 375 °F. Cook the pasta according to package directions. Reserve 1/4 cup of the cooking liquid, drain the pasta and return it to the pot. Meanwhile, heat 1 Tbsp oil in a large skillet over medium-low heat. Add the onion and 1/4 tsp each salt and pepper and cook, covered, stirring occasionally, until very tender, 6 to 8 minutes. Stir in the garlic and cook for 1 minute. Whisk in the milk and bring to a simmer. Whisk in the cream cheese, Cheddar, Parmesan and nutmeg and simmer, stirring occasionally, until slightly thickened, 2 to 3 minutes. Toss with the pasta, adding some of the reserved cooking liquid if the pasta seems dry. In a food processor, pulse the bread until coarse crumbs form. Transfer to a rimmed baking sheet and toss with the remaining Tbsp oil and 1/4 tsp each salt and pepper. Bake until golden brown, 4 to 6 minutes; toss with the scallions. Serve over the pasta.