Place a large cast-iron skillet over high heat, when very hot, add the oil and sear one of the racks all over until it is very brown, about 2 minutes per side.
Transfer to a roasting pan fitted with a rack, laying it rounded side up.
Repeat with the remaining rack.
Cover with foil and set aside.
Place the remaining ingredients, except the vinegar, in a food processor.
Add 1/2 tsp salt and 1/4 tsp pepper, and process until it forms a paste.
Press the paste onto the racks.
Roast until desired doneness, 15 min for medium rare.
Allow to rest for 5 min, covered, then carve and serve drizzled with balsamic vinegar.