Peel and chop the onions. Cut the ham into strips. Heat oil in a pot. Brown the ham for about 2 minutes. Add the onions, season with salt and pepper and continue to fry for about 2 minutes.
Dust with flour, sweat briefly and deglaze with cream, milk and stock. Let the sauce simmer for 2-3 minutes. Meanwhile wash the chervil, dab dry and remove coarse stalks. Chop leaves and fold into the sauce.
Season to taste with salt, pepper and lemon juice.