Sauce: Saute onions and celery in oil over medium-high heat until soft. In a smaller saucepan, mix sugar and vinegar and make over high heat about 3 min. Add orange juice to sugar mixture form, make 1 min with and set aside.
Add garlic to onions and cook 2 min. until soft, then add sugar mixture and cook as well. Add the remaining ingredients and gently simmer for 20 min. Whip in a hand mixer for 5 min. until creamy. The mixture makes about 1 liter of sauce. About 1/4 (250 ml) is needed for the recipe, the rest can be frozen in portions.
Grill: Coat the meat with the sauce about 1 hour before grilling. Grill at high temperature about 25 cm above the fire, turning not too often to the other side so that the sauce caramelizes. Remove from the broiler when the meat is still pink inside, rest for 2-3 minutes and then cut into 1/2 cm thick slices.