Peel and wash the potatoes, cover and cook in boiling salted water for about 20 minutes. Peel and finely dice onions. Melt 80 g butter in a pot. Steam onions for about 3 minutes until translucent.
Dust with flour and sweat for 1-2 minutes. Add wine, 600 ml water and 750 ml milk while stirring. Stir in stock, bring to the boil and simmer for about 5 minutes. Season to taste with salt and pepper.
Cut broccoli into florets, wash and possibly cut smaller. Blanch in boiling salted water for about 3 minutes. Drain and rinse with cold water. Rinse salmon, dab dry and dice. Season with salt.
Grate the cheese roughly. Drain the potatoes, add 400 ml milk and 80 g butter and mash with a potato masher until mashed. Stir in cheese, up to 8 tablespoons. Season to taste with salt and nutmeg.
Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Grease two casseroles. Carefully stir the fish and broccoli into the sauce. Spread over the moulds. Halve the purée and distribute in the same way.
Sprinkle each mould with 4 tablespoons of cheese. Bake in a hot oven for about 30 minutes.