Peel and wash the potatoes and cut them into cubes of about 1 cm. Heat 2 tablespoons of oil in a coated pan, fry the sweet potatoes in it at medium heat for about 10 minutes. Clean, wash and halve the Brussels sprouts.
Cook in boiling salted water for 8-10 minutes. Drain and let drain well. Peel shallots and cut into fine rings. Add to the potatoes shortly before the end of the frying time.
Let the dough sheets rest at room temperature for about 10 minutes. Wash the thyme, shake dry and chop finely. Heat 2 tablespoons of oil in a pan and fry the minced meat until crumbly. Mix minced meat and thyme with Brussels sprouts and sweet potatoes, season with cinnamon, salt and pepper.
Melt the fat and brush an ovenproof dish (28 cm Ø) with it. Spread each sheet of dough individually with fat and place it in the baking tin in an offset position. Fold over protruding edges to the inside. Whisk yoghurt and eggs and mix with the vegetable mince mixture.
Distribute into the mould. Bake in the preheated oven (electric oven: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) on the middle shelf for about 35 minutes. Cover with foil about 15 minutes before the end of the baking time.
Stir sour cream and lemon juice until smooth, season with salt and pepper. Remove from the oven and serve with sour cream.