Preheat oven to 425 degrees.
Combine garlic powder, onion powder, 1/4 teaspoon dried thyme, black pepper, cayenne pepper, and salt in a small bowl.
Rub seasoning mixture onto pork tenderloin, distributing evenly. Set pork aside.
Mix sliced turnips and sweet potatoes in large bowl with 2 tablespoons of vegetable oil, and 1/2 teaspoon dried thyme (or 1 1/2 teaspoons fresh thyme leaves). Season with salt and black pepper. Transfer to large baking dish.
Bake for 30 minutes, stirring halfway through to ensure even cooking and to coat vegetables with oil.
During last 15 minutes of vegetable baking time, heat remaining 1 tablespoon of vegetable oil in large skillet over medium-high heat. Add pork to skillet and brown evenly on all sides. Remove skillet from heat.
Remove turnips and sweet potatoes from oven and push with a spatula to arrange them along sides of baking dish. Place pork tenderloin in middle of baking dish, surrounding it with vegetables.
Mix butter, molasses, and remaining 1/2 teaspoon dried thyme (or 1 1/2 teaspoons fresh thyme) with drippings in skillet. Stir, allowing butter to melt while scraping up any tidbits on skillet bottom. Add whiskey to molasses mixture and stir to combine.
Drizzle molasses mixture over pork tenderloin and vegetables.
Return baking dish to oven and roast for 15-20 minutes, until desired doneness is reached (145 degree minimum internal temperature for medium, 165 degrees for well done).
Remove baking dish from oven. Transfer pork to cutting board and cover with foil. Allow to rest for 5 minutes.
Slice pork. Arrange on plates with vegetables and serve.