Set the sous vide to your desired temperature according to the chart below (I prefer 131-degrees for medium-rare).
Season lamb with black pepper and top with garlic.
Vacuum seal the lamb using your preferred method. 1 rack per bag.
Cook for 1 hour, up to four hours***.
Remove lamb, discard garlic and pat dry with paper towels. Season with salt and pepper.
Preheat a cast iron skillet over high heat. Working 1 rack at a time, add 1 Tbsp. oil, 1 Tbsp. butter, 1 sprig rosemary and 1 sprig thyme. Sear lamb 1-minute per side, tilting skillet and spooning butter over it as it browns. Remove to a cutting board and repeat with remaining lamb rack, oil, butter, and herbs. Slice each rack into 4 double chops and serve.
Rare: 115-124 degrees (very red inside)
Medium-Rare: 125-135 degrees (bright pink inside)
Medium: 135-144 degrees (light pink inside)
Medium-Well: 145-154 degrees (barely any pink left)
Well-Done: 155 degrees + (no pink)***For food safety reasons, when cooking at a temperature of 130-degrees or less, your food should spend a maximum of 2 ½ hours in the water bath.