Million Dollar Spaghetti

For the Meat Sauce - In a Dutch oven or large pot, heat oil over medium heat until shimmering. Add onion and cook, stirring often, until softened, about 8 minutes. Add beef and sausage and cook, stirring often to break up meat, until no longer pink, about 8 minutes. Add garlic, tomato paste, fish sauce, oregano, bay leaf, salt, black pepper, and crushed red pepper and continue to cook, stirring constantly, until evenly combined and fragrant, 1 to 2 minutes. Stir in tomatoes. Pour water into empty tomato can; swirl and pour into Dutch oven.

For the Meat Sauce - Bring to a simmer over medium heat. Reduce heat to low; simmer, uncovered, stirring occasionally, until sauce is thickened and flavors develop, 30 to 40 minutes. Remove and discard bay leaf, and season to taste with additional salt and black pepper, if desired. Set aside and keep warm until ready to use.

For the Béchamel - Meanwhile, in a wide 3-quart saucepan, melt butter over medium-high heat. Add flour and whisk to form a paste, then cook, stirring until the raw flour smell is gone, about 1 minute. While whisking constantly, add milk in a thin, steady stream, whisking thoroughly and getting into all corners of the pan to maintain a homogeneous texture. Cook, stirring often, until sauce comes to a simmer and begins to thicken slightly. Reduce heat to low and continue simmering, stirring often, until the sauce is thick enough to coat the back of a wooden spoon, 4 to 6 minutes. Remove from heat.

For the Béchamel - Gradually stir in mozzarella and Parmesan until melted. Sprinkle with nutmeg, salt, and pepper. If the cheese is struggling to melt, return the saucepan to low heat and whisk vigorously until melted. If any lumps form, simply whisk thoroughly to smooth out sauce. Use sauce right away, or press a piece of plastic wrap over surface of sauce to prevent a skin from forming and keep warm until ready to use.

For the Spaghetti - Bring a large pot of salted water to a boil over high heat. Add spaghetti and cook according to package directions until 1 minute shy of al dente; drain. Transfer the spaghetti to a bowl, and toss with butter until melted, then toss with 3/4 cup béchamel until evenly coated.

To Assemble - Adjust oven rack to middle position and preheat oven to 350°F (175°C). Lightly coat a broiler-safe 13- x 9-inch baking dish with cooking spray. Spread spaghetti mixture evenly in prepared dish; spread meat sauce evenly over top of spaghetti. Spread remaining béchamel evenly over top of meat sauce; sprinkle evenly with Parmesan (baking dish will be very full). Place baking dish on a large rimmed baking sheet to catch any drips.

To Assemble - Bake, uncovered, until heated through, bubbling around edges, and top is golden brown, 30 to 40 minutes. (If spaghetti is bubbling but needs more color on top, preheat oven to broil with rack about 7 inches from heat source. Broil until top is browned, about 2 minutes.) Remove from oven; let stand for 10 to 15 minutes before slicing and serving.