Whisk eggs, 1 pinch of salt, 375 ml milk and mineral water. Mix 200 g flour and baking powder and stir in well. Leave to swell for about 20 minutes. In the meantime, clean the leek, cut in half lengthwise and wash, cut into strips. Blanch leek in boiling salted water for 2-3 minutes, drain, rinse with cold water and drain.
Cut turkey breast into strips as well. For the sauce, melt butter, dust with 20 g flour and sauté while stirring. Gradually stir in 250 ml milk and 250 ml water. Bring to the boil, stir in broth and simmer for about 5 minutes. Heat oil in portions and bake 7 pancakes from the dough one after the other. Wash the chervil, dab dry and cut finely. Spread leek, turkey breast and chervil on the pancakes and roll them up. Cut each pancake into 3 pieces. Place the pancake rolls in a greased casserole dish (22 cm Ø; 1.3 litres capacity). Season the sauce to taste with salt and pepper and pour over the casserole.
Wash the chervil, dab dry and cut finely. Spread leek, turkey breast and chervil on the pancakes and roll them up. Cut each pancake into 3 pieces. Place the pancake rolls in a greased casserole dish (22 cm Ø; 1.3 litres capacity). Season the sauce to taste with salt and pepper and pour over the casserole. Sprinkle with cheese. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 20 minutes