To craft the most mouthwatering lasagna ever, start by preheating your oven to 375°F (190°C). Begin by cooking your pasta according to package directions until al dente. Meanwhile, create a rich meat sauce by browning ground beef in a skillet, adding onion and garlic for added flavor, then mixing in canned tomatoes and tomato paste. Next, prepare the cheese mixture: combine ricotta with egg, salt, and pepper. Stir in grated Parmesan and mozzarella until well combined. In a separate bowl, mix softened butter with flour to create a smooth roux. Add milk and bring to a simmer before removing from heat. Now it’s time to assemble our masterpiece! Spread a thin layer of the meat sauce on the bottom of a 9×13-inch baking dish, followed by a layer of cooked lasagna noodles. Spoon over the cheese mixture, then dot with butter roux. Repeat this process two more times, finishing with a layer of melted mozzarella on top. Place the dish in the oven and bake for 35-40 minutes or until the cheese is golden brown and bubbly. Remove from heat and let it rest for at least 10 minutes before serving.