Wash the potatoes and cook covered in plenty of water for about 20 minutes.
In the meantime peel the onion and cut it into fine slices. Clean, wash, spin dry and cut into strips. Wash chervil, shake dry and pluck the leaves from the stems.
Heat butter in a pot. Sauté the onion in it. Dust with flour and sauté briefly. Stir in 1/8 l water, milk and stock. Bring to the boil, simmer for about 5 minutes. Add frozen peas to the sauce and cook for about 3 minutes.
Season to taste with salt and pepper.
Drain the potatoes, rinse with cold water and peel. Add lettuce and chervil to the peas. Roll up ham slices and arrange with potatoes and pea vegetables.