Clean and sanitize the mushrooms. Break out or cut the stem at the base and chop finely. Heat 2 tablespoons of oil in a large frying pan. Fry the mushrooms in it, possibly in portions on both sides for 4-5 minutes until brown, take them out and let them drain on kitchen paper.
Cut the ham into small pieces and fry it in a hot frying pan for 3-4 minutes, turning it over and taking it out. Peel and finely dice the onions. Fry the onions and mushroom stems in the hot frying pan for 4-5 minutes, turning, until all the liquid has evaporated.
Wash parsley, dab dry, pluck leaves from the stalks and chop finely. Stir ham and parsley into the onions, season with salt and pepper, let cool down a little. Clean, wash and drain the rocket.
Season the vinegar with salt, pepper and sugar, fold in 2 tablespoons of oil. Fill mushroom heads with ham and parsley mixture. Arrange rocket on small plates and sprinkle with vinaigrette. Arrange 3-4 stuffed mushrooms on each plate.