Place the mayonnaise, onion slices, parsley and freshly ground pepper in a bowl. Combine them with a spoon, add the meat and give it a good toss. Cover with cling foil, put in the fridge and leave to marinate for 2h or, better still, for a whole day, turning everything a few times during that period. Then peel the onion, cut it into 4 and then halve each to end up with 8 pieces.  Halve and deseed the peppers, halve them again and then cut each half into 4. Finally halve the cherry tomatoes.  Now assemble the kebabs by sliding an onion on to a skewer, followed by some meat, a tomato, pepper and onion. Keep alternating the ingredients finishing off with a vegetable. When ready place all kebabs on a large serving plate and season with salt and pepper. Grill them for 3-4 minutes on each side. They usually take 12-15 minutes to cook through. For the roast potatoes: Peel, rinse and cut each potato into 6. Place them in a casserole, cover them with water, bring to the boil and cook them for 15 minutes. Poke them with a fork to check if they are ready. In the meantime preheat the oven to 250ºC. Empty the water from the casserole and give it a good shake so that the potatoes break into irregular pieces. Array them on a baking tray, drizzle them with olive oil, season with salt and pepper, sprinkle them with oregano and give them a good toss. Roast them for 30-35 minutes until golden brown, crispy on the outside and mellow on the inside.