Stir until all combined, then simmer for 10 minutes. Break the lasagne sheets into 2–4 pieces each and push them down into the sauce. Put them in randomly, not all on top of each other.
Stir in the cheese, mustard, nutmeg and ¼ teaspoon salt and season to taste with black pepper. Add the pasta and vegetables to the sauce and stir well until coated. Serve hot.