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1
Season lamb generously with salt and pepper on all sides. Place inside two gallon-sized zipper lock bags along with optional aromatics (for particularly large racks, split into 4 four-rib sections and use more bags as necessary). Squeeze out as much air from bags as possible with hands and close zipper lock, leaving one-inch un-sealed. Slowly submerge lamb in large pot of water, until only sealing edge of bags is exposed. Any remaining air should be forced out of bag as it is submerged. Seal bag completely. Repeat with remaining bags, then set all bags aside.
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2
Heat at least two gallons of water on the stovetops to 130 degrees for medium rare or 145 degrees for medium, using instant-read thermometer to ensure temperature accuracy (in some homes, the hot tap may be hot enough without having to use the stovetop). Pour water into picnic-size chest cooler. Add water until cooler is filled with water at desired temperature, sealing cooler in between additions in order to retain heat. Add lamb to cooler, seal, and set it warm spot for at least 45 minutes, and up to three hours, checking temperature of water after 15 minutes to ensure that it isn't losing heat too rapidly (water should lose at most 1 degree of temperature after 15 minutes-if it has lost more, top up with boiling water, and drape cooler with several bath towels to help insulate.
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3
Remove lamb racks from bags, discard aromatics (if using), and pat dry with paper towels. Heat oil in heavy-bottomed 12-inch stainless steel, cast iron, or non-stick skillet over high heat until lightly smoking. Add lamb to skillet fat-side down and sear until well-browned on all sides, turning with tongs as necessary, about 5 minutes total (for particularly large racks, sear lamb in two batches, tenting first batch with foil after searing to retain heat). Tent seared lamb with foil, allow to rest 5 minutes, carve, and serve.
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4
Light large chimney starter 3/4 filled with charcoal (4 quarts, about 75 briquettes), and burn until coals are coated in thin layer of gray ash, about 20 minutes. Empty coals and spread evenly over half of grates. Position grill grates on top of grill, cover grill, and heat until grate is hot, about 5 minutes. Scrub grill clean with grill brush if necessary. Meanwhile, remove lamb racks from bags, discard aromatics (if using), and pat dry with paper towels. Rub lamb racks evenly with oil. Place racks, fat-side down over coals and grill until well browned on all sides, about 6 minutes total, moving lamb to cooler side of grill as necessary to avoid flare-ups. Remove lamb from grill, tent with foil, allow to rest 5 minutes, carve, and serve.
