To make turnip fries, cut the turnips in half lengthwise. Cut each half into 12 inch slices. Heat a large skillet over medium-high heat. Add 1 tablespoon of olive oil to the skillet and saute the onion until softened, about 5 minutes. Stir in the garlic and cook, stirring, until fragrant, 1 minute. Remove the pan from the heat and add the potatoes. Cook, tossing, for 3 to 4 minutes or until potatoes are tender. Season with salt and pepper to taste. Serve with ketchup and sour cream.