Combine the marinade ingredients in a medium bowl and whisk to thoroughly combine. Reserve 3 tablespoons of the mixture and set aside in a small bowl.
Place the whole rack of lamb in a large resealable plastic bag, and pour the marinade you haven’t set aside over it, tossing and rubbing to make sure the lamb is evenly and well coated. Roll the bag closed to get the air out and seal the lamb in with the marinade.
Place the bag in the refrigerator and marinate at the very least two hours and preferably overnight.
Preheat the oven to 275°F. Toss the fennel and baby potatoes with one tablespoon of the remaining marinade and arrange them in the bottom of a baking dish just big enough to fit the lamb along with the vegetables.
Meanwhile, heat a skillet over high heat on the stove. Remove the lamb from marinade, wiping off excess & drying the meat. Coat the bottom of a skillet big enough to fit the lamb easily with a thin coating of grapeseed oil and sprinkle the lamb with kosher salt.
When the skillet is searingly hot, brown the lamb on all sides using tongs when necessary. It should only take about a minute or two for each side to have a deep, golden brown crust. The goal is to get a nice, flavorful crust without cooking the lamb internally which is why it’s essential that your skillet be very hot.
As soon as it’s brown, remove the lamb from the skillet and place over the potatoes and fennel in the baking dish (I use, unsurprisingly, a cast iron skillet for this.)
Drizzle the seared lamb with the remaining reserved two tablespoons of marinade, and roast about 25-35 min or until lamb reached internal temp of 120°-125°F on a meat thermometer for medium-rare.
While the lamb roasts, make the chimichurri. Mince the parsley, fennel and mint.
In a medium bowl, combine the herbs with the garlic, salt, lemon juice, honey and red pepper. Stir to combine well until the salt is dissolved. Drizzle in the olive oil and stir. It will keep for several days covered in the fridge, and is excellent on pretty much any and all roasted or grilled meats and veggies.
Remove the lamb from the oven, and let it rest loosely covered in foil for 15 minutes (it continues to cook while it rests). Once the lamb has rested, remove it to a carving board and slice it between the bones. Serve it atop the roasted veggies with Mint Fennel Chimichurri.