Clean, wash and quarter the mushrooms. Peel and finely chop the onion. Fry in hot oil. Add mushrooms. Season with salt and pepper. Fry for approx. 5 minutes, turning the mushrooms. Then drain.
Wash the tomatoes, cut out the stem base. Cut the tomatoes into slices. Cut ham slices in half. Pour prepared vegetables and ham into an ovenproof dish. Whisk eggs and milk. Season with salt and pepper.
Pour over the vegetables. Sprinkle with cheese. Bake in the preheated oven (electric cooker: 225 ° C/ gas: level 4) for about 15 minutes. Cover if necessary in between. Serve garnished with parsley leaves.