Separate eggs. Mix egg yolks and cream cheese. Sift the baking powder over it and stir in quickly. Beat the egg whites with the whisk of the hand mixer until stiff. Add 1 pinch of salt. Carefully fold approx. 1/2 beaten egg white into the egg yolk mixture, then fold in the remaining beaten egg white.
Spread the mixture with a tablespoon in 8 blobs on a baking tray lined with baking paper. Bake in a preheated oven (electric cooker: 150 °C/ convection oven: 125 °C/ gas: see manufacturer) for approx. 25 minutes.
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Peel and finely dice the onion and garlic. Knead minced meat, mustard, onion and garlic. Season with 1 tsp salt and pepper. Shape into 4 flat meatballs. Heat the oil in a large pan and fry the meatballs for about 4 minutes on each side.
Take the bread out of the oven and let it cool down briefly. Cover 4 flat cakes with lettuce, spread 1 tbsp. lettuce cream on each flat cake and layer the meatball, cheese, tomato, cucumber, red onion rings and remaining lettuce cream on top.
Sprinkle with chives and place the rest of the bread on top. If necessary, fix it with wooden skewers.