Mix the eggs, 250 ml milk, 1 pinch of salt and 200 g flour with a whisk to a smooth dough. Cover and let it swell for about 15 minutes. Roast the walnuts in a pan without fat. Remove and chop coarsely, except for a few for garnishing.
Clean, wash and quarter the radicchio. Remove the stalk and cut the quarters into strips. Heat 1 tablespoon of oil in a large frying pan. Fry the radicchio strips in the pan for about 1 minute, then add vinegar and sugar and mix well.
After 1-2 minutes add chopped walnuts and remove from the heat. Season with salt, pepper and possibly some sugar. Heat 2 tablespoons of oil in portions in a frying pan (bottom approx. 20 cm Ø). Bake 8 pancakes from the dough one after the other.
Cover each pancake with 1 slice of ham and radicchio, then roll up tightly. Place the crespelle in a greased casserole dish (approx. 15 x 20 cm). Wash lemon hot, grate dry and thinly grate half of the peel.
Halve the lemon and squeeze the juice. Melt the fat in a small pot. Stir in 20 g flour and sweat for about 1 minute. Add 250 ml milk, stock and lemon juice while stirring and simmer gently for 3-5 minutes.
Grate parmesan. Add lemon zest to the sauce and season with salt, pepper and a little nutmeg. Spread the lemon sauce over the crespellet and sprinkle with parmesan. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 15-20 minutes.
Sprinkle with remaining walnuts.