Cook the pasta in boiling salted water according to the manufacturer's directions and transfer to a bowl of ice water. Drain the noodles and stack on a baking sheet with a paper towel between each layer. Set aside.
Preheat oven to 350 degrees. Grease a 13 x 9 inch baking dish with butter to prevent sticking.
In a large saucepan, heat the butter over medium heat until foaming. Whisk in the flour and cook, stirring constantly for 1 minute. Slowly whisk in the milk, 1/2 a cup at a time. Bring to a boil and then reduce heat to low. Simmer over low heat, whisking often. Cook until smooth, creamy, and thick enough to coat the back of a spoon, approximately 10 minutes. Remove from heat and let cool to room temperature, stirring occasionally.
Add the bolognese sauce to a large saucepan and reheat until warmed through.
Spread 1/4 cup of the bechamel sauce in the bottom of the baking dish. Top with a layer of noodles and spread 3/4 cup of the bolognese over the noodles.
Top the bolognese with 1/2 cup bechamel and 1/4 cup of parmesan. Repeat the process starting with the noodles and ending with the Parmesan until you have 8 layers. Be sure to layer the noodles, bolognese, béchamel, parmesan, and mozzarella evenly so every slice has balanced flavor and texture. Arrange the mozzarella slices on the top of the lasagna.
Place the baking dish, covered with foil, on a baking sheet and bake the lasagna for approximately 30 minutes.
Remove the foil and bake for an additional 20-30 minutes. Bake until the top is golden brown in spots and the sauce is bubbling around the edges.
Rest the lasagna for 15 minutes before slicing and serving.
Notes
Let the béchamel cool slightly before assembling so it spreads more evenly.
Drain the noodles well before layering to prevent extra moisture.
A thicker bolognese will give you cleaner, sturdier slices.
Rest the lasagna before serving so the layers hold together.
Fresh mozzarella gives a softer, richer finish, while low-moisture mozzarella creates a firmer top.