Mix 1 egg, 200 ml milk and flour to a smooth dough, season with salt and pepper, let rest for a while. In the meantime clean and wash the broccoli and divide into small florets. Cook in boiling salted water for about 5 minutes and drain.
Peel onions and cut them into rings. Drain mozzarella and cut into slices. Wash the dill, shake dry and pluck the flags. Fry the dough in a frying pan (approx. 20 cm Ø) with a little oil in each case to make 8 very thin pancakes.
Fry the onion rings briefly in the pan and drizzle with vinegar. Spread salmon and dill on the pancakes. Season with pepper and roll up. Spread the broccoli in an ovenproof dish. Place rolled pancakes on top and spread the onion rings on top.
Whisk the sauce, 100 ml milk and 2 eggs. Season with salt and pepper and pour over the casserole. Cover with mozzarella slices and bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 15-20 minutes.