Peel, wash and slice the potatoes. Pumpkin clean, wash, dry grate, halve, core and cut into slices. Zucchini wash, dry grate and cut into slices.
Wash the parsley, dab dry and pluck the leaves from the stalks. Chop parsley finely, except for a few leaves for garnishing. Peel and finely chop the onion. Leave the bacon out briefly in a pot (not quite crispy), take it out and let it drip off on kitchen paper.
Add fat to the bacon fat, melt. Sauté the onion cubes in it. Sprinkle with flour and sweat. Add stock, milk and cream while stirring. Stir in mustard and chopped parsley. Season to taste with salt and pepper.
Bring to the boil and simmer for 1-2 minutes. Meanwhile grate cheese. Grease an oven dish. Layer potatoes, vegetables and bacon like roof tiles. Pour béchamel over it. Sprinkle with cheese and mustard seeds.
Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 45 minutes. Serve garnished with remaining parsley.