Preheat oven to 375°F. Coat large ovenproof dish with butter. Set aside.
Bring large pot of generously salted water to a boil. Add pasta and cook until just shy of al dente. Drain and set aside.
While pasta is cooking, melt butter over medium heat in large saucepan. Add flour and stir with wooden spoon until paste forms. Gradually whisk in milk. Season with salt, pepper and nutmeg. Reduce heat to low and continue cooking until sauce thickens, stirring as necessary.
Stir in mozzarella and Bolognese sauce until combined. Add pasta and stir until evenly coated. Pour mixture into prepared baking dish. Sprinkle top with parmesan and bake until golden and bubbly. Approximately 25 minutes.