Cook the lasagna noodles.
Sauté the onion in 2 tbsp of butter. Add the flour and stir. Add the milk, stirring with a whisk. Stir until the sauce thickens. Season with salt and pepper to taste.
Add the mixed vegetables to the mixture and stir.
Spread the noodles in a dish, cover with sauce. Add more noodles, then more sauce. Also spread the cheese and bake at 425°F.
Remove from the oven when the cheese is golden brown.