Wash the meat, dab dry and cut a pocket. Peel the onions and cut half into slices. Chop the rest of the onions finely. Cut toast into very small cubes as well.
Mix onion cubes, toast, mustard and egg and fill into the meat opening. Plug with wooden skewers and wrap with kitchen string. Season meat all around with salt and pepper. Place on the fat pan of the oven.
Cook in the preheated oven (electric cooker: 200 °C/ gas: level 3) for about 1 hour. Add the onion slices after half the frying time. Deglaze with broth about 15 minutes before the end of the frying time and braise to the end.
In the meantime, peel and wash the potatoes and cut them in half lengthwise. Cook in boiling salted water for 15-20 minutes. Wash and clean the zucchini and tomatoes. Cut zucchini into pieces, cut tomatoes into eighths. Wash thyme, dab dry, pluck leaves from stems and chop.
Shortly before serving, fry the zucchini in hot oil while turning for about 3 minutes. Add tomatoes and cook for another 5 minutes. Season with salt, pepper and thyme. Fold in potatoes. Remove roast from the oven.
Pour the roast stock into a pot and bring to the boil. Add sauce thickener while stirring and bring to the boil again. Season to taste with salt and pepper. Refine with crème fraîche. Cut the roast open, arrange with vegetables and sauce.
Serve garnished with parsley.