1. Heat oven to 400 degrees F and place rack in lower third.
2. Rinse ribs and pat dry. Place each rack of ribs on a sheet of heavy-duty (or double layer) foil large enough to wrap around ribs. Sprinkle ribs with 4 tsp rosemary, 2 tsp salt, 2 tsp garlic and 1 tsp pepper flakes; wrap in foil, making an airtight seal. Transfer to a rimmed baking sheet and roast 1 hour and 30 minutes.
3. Meanwhile, for glaze, in a small bowl, whisk together marmalade, vinegar, remaining 2 tsp rosemary, 1/2 tsp salt, 1/2 tsp garlic and 1/2 tsp pepper flakes. Cover and set aside.
4. Heat grill to medium high. Lightly brush ribs with some glaze, remove foil and grill, turning occasionally, until grill-marked and golden brown, 8 to 10 minutes. Generously brush with remaining glaze, slice and serve garnished with rosemary sprigs, if desired. Makes 4 to 6 servings.