The night before,smash garlic with the back of a knife and smoosh in 1 tablespoons of kosher and thyme leaves until you have a paste. Add the mustard and spread mixture over the pork loin. Store overnight in the refrigerator. Allow roast to come to room temperature before cooking.
Preheat oven to 425 degrees.
Toss carrots, potatoes, and onion slices in large bowl with the olive oil, melted butter, and salt and pepper to taste. Place vegetables in large roasting pan and roast in oven for 30 minutes.
Add pork roast to roasting pan and roast for 50 minutes, or until meat thermometer inserted in middle of roast reads 140 degrees. Remove from oven, cover meat with foil and allow to rest for 15 minutes.
Cut strings from roast and serve on a platter with roasted veggies.