Step 1: Prepare the Meat Sauce: In a large pot or Dutch oven, heat olive oil over medium-high heat. Add ground beef and Italian sausage, breaking it up with a spoon. Cook until browned, about 8-10 minutes. Drain any excess fat. Add chopped onion and garlic, cooking until softened, about 5 minutes. Stir in crushed tomatoes, tomato paste, dried oregano, dried basil, bay leaf, salt, and black pepper. Bring to a simmer, then reduce heat to low, cover, and let it gently simmer for at least 1 hour, stirring occasionally, or up to 2 hours for deeper flavor. Remove the bay leaf before using.
Step 2: Make the Béchamel Sauce: In a separate medium saucepan, melt butter over medium heat. Whisk in flour and cook for 1-2 minutes, stirring constantly, to create a roux. Gradually whisk in warm milk, a little at a time, to avoid lumps. Continue whisking until the sauce thickens and is smooth, about 5-7 minutes. Season with salt, white pepper, and a pinch of nutmeg. Remove from heat.
Step 3: Cook the Lasagna Noodles: Bring a large pot of salted water to a rolling boil. Add the lasagna noodles and cook according to package directions until al dente. Do not overcook. Drain thoroughly and lay them flat on a baking sheet lined with parchment paper or wax paper to prevent sticking.
Step 4: Assemble the Lasagna: Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish. Spread a thin layer of meat sauce on the bottom of the dish. Arrange a single layer of cooked lasagna noodles over the sauce, slightly overlapping if necessary. Spread about one-third of the béchamel sauce over the noodles. Top with one-third of the remaining meat sauce, then sprinkle with a generous portion of mozzarella and Parmesan cheeses. Repeat the layers: noodles, béchamel, meat sauce, cheeses. Finish with a final layer of noodles, the last of the béchamel, and a thick layer of mozzarella and Parmesan cheese.
Step 5: Bake the Lasagna: Cover the baking dish loosely with aluminum foil (you can spray the underside of the foil with cooking spray to prevent cheese from sticking). Bake for 45 minutes. Remove the foil and bake for an additional 15-20 minutes, or until the top is golden brown and bubbly. If the top browns too quickly, you can cover it again loosely.
Step 6: Rest and Serve: Once baked, remove the lasagna from the oven and let it rest for at least 15-20 minutes before slicing and serving. This allows the layers to set and prevents a messy collapse. Enjoy!