Peel and finely dice the onions. Clean, wash and halve the cauliflower. Set one half aside. From the other half cut florets from the stalk. Dice stalk roughly. Heat the oil in a large pan.
Coarsely crumble the minced meat into it. Fry vigorously while turning. Season well with salt and pepper. Add cauliflower and onions and fry briefly.
Add 25 g butter and melt. Dust with flour, sweat on, deglaze with stock and cream. Boil up while stirring. Stir in mustard. Season to taste with Worcestershire sauce, salt and pepper.
Pull off the stove.
For the bottom grate the cheese finely. Finely grind the second cauliflower half in portions in the universal chopper. Peel potatoes, wash thoroughly and grate coarsely. Squeeze the potatoes with your hands.
Knead 75 g cheese, cauliflower crumbs, 50 g butter, egg, potato flour, potatoes, salt and pepper well. Pour the dough into a greased tart tin (28 cm Ø) with lift-off base, sprinkled with breadcrumbs.
Press the mixture on the bottom and the edge of the mould with your hands (it sticks a little).
Sprinkle with breadcrumbs. Spread the chopped cauliflower mixture evenly on top. Sprinkle with remaining cheese. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for approx. 50 minutes on the bottom shelf.
Cover with foil about 10 minutes before the end of the baking time.
Wash parsley, shake dry, pluck leaves from the stalks and chop. Remove the tart from the oven and place it on a cake rack. Let it rest for about 10 minutes. Remove from the tin, arrange on a plate and sprinkle with parsley.