Preheat oven to 425 degrees.
In a food processor, place garlic, thyme leaves, and 1 tablespoon kosher salt. Grind and add Dijon mustard and pulse. Spread the mixture on the top (fat side up) of the roast and allow to sit at room temp for about 30 minutes. Insert a probe programmable thermometer and set to 138 degrees.
Meanwhile, cut the vegetables. Remove the tops of the fennel and slice a thin layer of root off the bottom keeping the root intact. Then wedge into 8 pieces cutting through the core so the wedges stay together. Toss all the vegetables in the roasting pan with the olive oil, melted butter, and salt and pepper to taste. Cook for 30 minutes. Remove the pan, add the pork on top of the vegetables, and continue to roast another 30-50 minutes, or until EXACTLY 138 degrees internally.
Remove from the oven, cover with foil and let rest 15 minutes. The Butcher’s Wife puts a clean towel on top of the tin foil to help keep it warm. Remove the strings and slice ¾” thick. Arrange the meat and vegetables on a platter. Sprinkle with kosher salt and pepper, and garnish with fresh thyme.