For the cabbage lasagna on mustard-beer sauce, first peel and finely chop the onion, heat 2 tablespoons of oil in a pot and fry the onion until golden. Caramelize with sugar. Add cabbage and roast well. Pour in soup and season. Simmer on low heat for about 15 minutes. Stir frequently.
Heat butter in the pot, mix with flour and pour in milk. Season with salt, white pepper and tarragon. Add nutmeg if necessary. Stir in the cheese.
Butter the casserole dish and alternate layers of lasagna sheets, cabbage and béchamel sauce. Finish with béchamel sauce.
Bake lasagna in preheated oven at 200 °C for about 30 minutes. Let rest a little before cutting.
For the mustard-beer sauce, peel and finely chop onion, clean root vegetables (celery, carrots, parsley and leek) and cut into small cubes.
Heat butter, fry onion until golden, add root vegetables and fry briefly. Deglaze with beer and simmer.
Puree with a hand blender. Pour through a sieve. Cut pepperoni in half, remove seeds and chop flesh. Add pepperoni and mustard to beer sauce and mix well (blend again if necessary). Season to taste with salt and white pepper. If sauce is too thin, thicken with flour. Serve cabbage lasagna on mustard beer sauce.