Onions peel, halve and in fine cubes cut. Wash 1 lemon and dab dry. Finely grate the lemon peel, halve the lemon and squeeze the juice. Wash fish, dab dry and cut into cubes.
Heat the fat in a pot, fry the onions and lemon peel in it. Add stock and cream, bring to the boil. Add peas. Stir starch with 4 tablespoons of water until smooth. Stir into the boiling sauce, bring to the boil briefly.
Season to taste with salt, pepper, nutmeg and lemon juice. Wash the lemon balm and shake dry. Pluck off leaves and chop finely, except for something to garnish.
Stir the fish and chopped lemon balm into the sauce, except for something to garnish. Cook the fish at low heat stirring several times for about 10 minutes. In the meantime, wash the remaining lemon, grate dry and cut into slices.
Arrange the ragout, sprinkle with lemon balm and garnish with lemon and balm.