Stir eggs, salt, milk, mineral water, flour and baking powder until smooth. Let the dough swell for about 10 minutes. Wash the asparagus, cut off the lower woody ends. Cook in boiling salted water for about 8 minutes, rinse with cold water, drain and put aside.
Heat a small amount of oil in a small pan (approx. 18 cm Ø). Add 1/8 of the pancake batter and bake on each side over medium heat for 2-3 minutes until golden brown. Take out and process the remaining dough in the same way.
For the hollandaise bag contents, stir into 125 ml cold water and cream. Bring to the boil briefly while stirring. Switch off the stove. Add butter in pieces and fold in until the butter has melted.
Remove from the hotplate immediately. Cover each pancake with 2 slices of salmon. Place about 3 stalks of asparagus on each pancake and roll up. Fix with wooden skewers. Put them into a greased casserole dish and pour the hollandaise sauce over them.
Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for about 15 minutes. Serve garnished with chervil.