Instructions

1. 5-ounce) can diced tomatoes in a heatproof colander in the sink and let sit in the colander (no need to rinse).Break 1 pound dried spaghetti into thirds with your hands and add to the boiling water. Cook until al dente, 8 to 9 minutes. Drain in the same colander over the tomatoes. While the spaghetti is cooking, start the sauce.Melt 4 tablespoons unsalted butter in a large Dutch oven or oven-safe pot over medium heat. Add 1 diced medium yellow onion, season with 1/2 teaspoon of the kosher salt, and cook until softened, 5 to 6 minutes. Stir in 1 diced medium red bell pepper and 3 minced garlic cloves and cook until the garlic is fragrant, about 1 minute.Stir in 1/4 cup all-purpose flour and cook, stirring often, until the flour films the bottom of the pot, 1 to 2 minutes. Increase the heat to medium-high. Stir in 2 cups whole milk, 1 cup low-sodium chicken broth, and the remaining 2 teaspoons kosher salt until smooth. Cook, stirring frequently and scraping the bottom of the pot, until it comes to a boil, about 4 minutes. Turn off the heat.Stir the tomatoes and spaghetti, 1 pound shredded, cooked chicken, and 8 ounces of the shredded sharp cheddar cheese (2 cups) into the sauce until evenly combined and the cheese until melted. Taste and season with kosher salt and black pepper as needed.Sprinkle the remaining 4 ounces shredded cheddar (1 cup) evenly over the top. Transfer the pot to the oven and bake until bubbling, about 20 minutes. For a browned top, turn the oven on to broil and broil until the cheese is browned in spots, 3 to 6 minutes. Let cool for 5 minutes before serving. Garnish with chopped fresh parsley leaves or chives if desired.Recipe NotesBaking dish: If you don’t have an oven-safe pot, make the casserole through Step 5, then transfer to a 9x13-inch baking dish. Proceed with Step
2. Make ahead: Assemble the whole casserole but do not bake. Let cool, then cover and refrigerate for up to 1 day. Let the casserole sit out at room temperature while the oven heats. Bake uncovered according to directions, adding 10 to 15 minutes to the bake time.Storage: Leftovers can be refrigerated in an airtight container for up to 4 days.