Step 1

Lightly score fat cap of each roast in a 1" crosshatch pattern, being careful not to slice into meat.

Step 2

Toast peppercorns and fennel seeds in a small dry skillet over medium heat, shaking pan occasionally, until fragrant and just beginning to smoke, about 4 minutes. Transfer to spice mill or mortar and pestle. Let cool slightly. Add red pepper flakes and cinnamon and finely grind.

Step 3

Using a skewer or paring knife, poke several holes into meat on all sides (this will help the seasoning penetrate; don’t be afraid to puncture the meat deeply). Season roasts with 3 Tbsp. Diamond Crystal or 5¼ tsp. Morton kosher salt, then rub ground spice mixture all over. Wrap tightly in plastic wrap and chill at least 8 hours.

Step 4

Unwrap roast and let sit at room temperature 1 hour before cooking.

Step 5

Place a rack in lower third of oven; preheat to 275°. Cook butter, one half head of garlic, and 1 thyme sprig in a small saucepan over medium-low heat, stirring often, until bubbling and fragrant but not browned, 5–6 minutes. Remove from heat and cover to keep butter warm.

Step 6

Toss leeks, potatoes, oil, and remaining garlic heads and 5 thyme sprigs in a large roasting pan; season with kosher salt. Place pork on top of vegetables and brush all over with some garlic butter. Roast, basting with garlic butter every 30 minutes or so and stirring vegetables, until an instant-read thermometer inserted into center of pork registers 120°, 75–105 minutes. Transfer pork to a cutting board. Tent pork with foil. Let rest at least 45 minutes or up to 3 hours.

Step 7

Check potatoes for doneness; they should give no resistance when poked with a fork (this will vary depending on the size of your potatoes). If they’re still firm, you'll need to cook them longer, along with the leeks and everything else in the roasting pan. Increase oven temperature to 500°. Cover pan with foil and roast until potatoes are tender and garlic is golden, 20–30 minutes longer (start checking after 10 minutes).

Step 8

If resting pork for the maximum 3 hours, turn off oven and remove foil from roasting pan. After meat has rested, preheat oven to 500°, add kale and vinegar to roasting pan, and toss to coat. Arrange pork on top of vegetables and roast until pork is deeply golden brown and an instant-read thermometer inserted into center registers 130°, 10–20 minutes (or possibly longer if you’ve rested your pork the full 3 hours). Let everything rest in pan 15–20 minutes.

Step 9

Transfer pork to a cutting board. Using tongs, transfer vegetables to a large platter, reserving pan juices. Carve meat off bones, then slice crosswise ½" thick. Arrange over vegetables. Spoon reserved juices from pan over meat (there’s a ton of flavor in them that shouldn’t be left behind!). Season with sea salt.

Step 10

Do Ahead: Pork can be seasoned 2 days ahead. Keep chilled.