Sweet and tangy pork tenderloin (with red potato mash)

Preheat oven to 500 degrees. Game plan: While oven is preheating, boil potatoes and prepare pork. While pork is cooking, mash cooked potatoes and mix with buttermilk and 1/4 teaspoon each salt and pepper. To prepare pork: Combine 1/2 teaspoon each salt and pepper, 1 teaspoon oil, thyme and garlic. Rub over pork. Place on foil lined jelly roll pan. Bake at 500 degrees for 10 minutes. Meanwhile, in a small saucepan, combine 1 teaspoon oil, jelly, vinegar and shallots. Bring to a boil and simmer for 1 minute. Remove from heat. Brush 1/2 of jelly mixture over pork and return to oven for 5 minutes. Turn pork over, cover with remaining jelly mixture and bake for 5-8 minutes more (or internal temperature of 145 degrees). Remove from oven and let rest for 5 minutes before slicing thinly on the cross-grain.