Peel and wash the potatoes and cook them in boiling salted water for about 15 minutes. In the meantime peel onion and cut into fine rings. Clean, wash and cut the peppers into bite-sized pieces. Wash the cherry tomatoes.
Cut the sausages into slices of about 1 cm thick. Heat the oil in a pan, fry the sausages for 2-3 minutes while turning them and take them out. Add onion and paprika to the oil and fry for about 3 minutes, turning. Season with salt and pepper and remove from the heat.
Cut the potatoes into slices and layer in an ovenproof dish (approx. 30 x 18 cm). Spread onion, paprika, cherry tomatoes and sausages on top. Whisk cream, eggs, mustard and paprika powder well together. Season to taste with salt and pepper. Pour the egg-cream mixture over the casserole, bake in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for approx. 20 minutes.
Wash the chives, shake dry and cut into fine rolls. Stir sour cream, lemon juice and half of the chives until smooth. Season to taste with salt and pepper. Take the casserole out of the oven and let it cool down briefly. Garnish with a blob of sour cream and chives, add the rest of the sour cream.