Mix eggs and 1/2 teaspoon salt in a bowl. Gradually stir in milk and flour. Finally, stir in oil and mineral water. Cover the pancake batter and let it rest for about 20 minutes.
Wash and peel the asparagus and cut off the woody ends. In a large saucepan, boil salted water with sugar and 10 g butter. Add the asparagus, bring to the boil and cook for about 15 minutes. Put some butter in a frying pan (20 cm Ø).
Add 1/8 of the pancake batter. Fry on both sides until golden brown. Wash the chervil, shake dry and, except for a little garnish, pluck the leaves from the stems and chop finely. Wash the lemon, grate dry and finely grate the peel.
Halve the lemon, squeeze half of it. Warm up Sauce Hollandaise, season to taste with lemon juice, zest and chervil. Lift asparagus out of the water, keep warm. Cover pancakes with ham and roll up.
Form the dough into a ball and put it back into the bowl. Cover and leave to rise for approx. 1 1⁄2 hours in a warm place. Knead the dough again with your hands. Then let it rest in the fridge for at least 3 hours.