Remove pork tenderloin from package and rinse with cold water, pat dry.
Place the tenderloin, fat side up in the center of a baking dish.
Cut baby potatoes in half and rinse in a colander.
Arrange the potatoes along the edges of the tenderloin.
Cut carrots into medallions and place on top of potatoes around the tenderloin.
Spread the garlic over the top of the tenderloin.
Salt and pepper the top of the tenderloin and the vegetables.
Thinly slice the onion and arrange on the top of the tenderloin.
Salt and pepper the onion slices.
Randomly drop pieces or tablespoons of the butter or margarine on the top of the pork loin and vegetables.
Tightly cover the roasting pan with aluminum foil.
Bake in the preheated oven for 2 hours.
Remove the aluminum foil, increase the oven temperature to 375F and cook for about 1 hour. Or until the potatoes and top of the tenderloin is golden brown and the center of the tenderloin is cooked through. Roasting time will depend on the size of the tenderloin.
Use pan drippings to make a gravy.
Recipe Expert Tips
Adjust roasting time for larger or smaller pieces of tenderloin.