Instructions

• Tear the toasted bread slices into roughly 2-inch pieces and pulse in a food processor until finely ground. Transfer the crumbs to a small bowl, add the 2 tablespoons melted butter, toss until evenly coated, and set aside.
• Adjust an oven rack to the middle position and preheat the oven to 400°F.
• Fill a large pot with water, bring it to a boil, and salt it. Break the 8 ounces spaghetti in half and add to the boiling water.
• Cook the spaghetti until al dente, about 7 to 9 minutes. Drain well and rinse with cold water to stop the cooking; set the pasta aside.
• Return the same pot to medium-high heat and add the 2 tablespoons butter. Melt until foaming.
• Add the finely chopped medium onion and cook, stirring occasionally, until softened, about 5 minutes.
• Add the minced garlic and minced fresh thyme and cook, stirring, until fragrant, about 30 seconds.
• Sprinkle in the 2 tablespoons all-purpose flour and cook, stirring constantly, until the flour is lightly golden, about 1 minute.
• Whisk in the 1 cup chicken broth and 1 cup half and half. Bring to a simmer and continue whisking until the sauce thickens, about 1 minute.
• Remove the pot from the heat and whisk in the 1/2 cup grated Parmesan until smooth. Season to taste with salt, freshly ground black pepper, and hot sauce if using.
• Add the cooked pasta, 2 cups cooked chicken (chopped or shredded), and 1 cup frozen peas to the pot and toss gently to combine and coat everything in the sauce.
• Transfer the mixture to a 2-quart baking dish and sprinkle the prepared breadcrumb topping evenly over the top.
• Bake until the sauce is bubbling and the topping is browned, about 10 to 15 minutes.
• Let the casserole rest for 5 minutes before serving.