Spread a thin layer of oil on the baking tray and line with baking paper. Preheat the oven (electric cooker: 200 °C/ circulating air: 175 °C/ gas: level 3), also heat the baking tray. Mix the buffer dough with egg.
Remove the hot baking tray and brush with 2 tablespoons of oil. Form 8 buffers from the buffer dough on the baking tray. Put back into the oven and bake for about 20 minutes, turning once. Rinse the maties, dab dry and cut into pieces.
Wash the cucumber and radishes and cut into pieces. Clean and wash spring onions and cut them into rings. Mix yoghurt and sour cream, season with salt, pepper and sugar. Wash the dill, shake dry and chop finely, except for something to garnish.
Stir into the yoghurt. Fold in radishes, spring onions, cucumber and matie. Arrange rösti and matie salad on plates. Garnish with dill and lemon wedges.