Heat up the grill. Wash the rosemary, dry it and put 1 spring under the skin of each chicken leg. Boil the potatoes. In the meantime, arrange the chicken legs on the grill (skin facing down). Smear them with olive oil, sprinkle with black pepper and salt. Grill about 10 min.
Turn the meat over, smear the other side with olive oil, salt and pepper and grill another 10 min. While you're grilling the chicken legs, pour the rest of the olive oil in a pot.
Mash the garlic and braise it in the pot. Turn the stove off. Beat the mayo with 2 tbsp hot water, cover with a lid and keep warm. Drain the potatoes and mash them.
Distribute the chicken legs and potatoes into 4 plates. Pour on the mayo, sprinkle with a bit more black pepper and serve.