For the crêpes, beat the eggs with the whisk of the hand mixer. Stir in flour. Stir in milk and mineral water. Season with salt and pepper. Leave to swell for about 20 minutes.
In the meantime, clean and slice the mushrooms very finely. Wash thyme, shake dry, put some stems aside for garnishing. Pluck the remaining leaves from the stems.
For the filling clean, wash and halve the tomatoes. Cut cheese into slices. Clean, wash and cut spring onions into small rolls. Clean, wash and drain lettuce well in a sieve.
Mix vinegar, salt, pepper and sugar. Fold in olive oil. Stir the thyme into the crêpe batter. Heat several portions of clarified butter in a pan (26 cm Ø). Put some mushroom slices in the pan, add just under a ladle (not too large) of batter.
Bake at medium heat from both sides until golden brown. Bake 8 crêpes one after the other with the remaining mushrooms and the remaining batter. Keep warm. Mix salad and marinade. Fold the crêpes into quarters and fill with tomatoes, cheese and spring onions.
Arrange 2 crêpes with some salad on plates. Sprinkle with pepper and garnish with thyme.