Combine sour cream and horseradish; cover and chill.
Combine potato, parsnip, and carrot; spread evenly onto several layers of paper towels. Sprinkle with 3/4 teaspoon salt; let stand 30 minutes. Cover with additional paper towels, and press down to absorb liquid. Place parsnip mixture in a large bowl. Add onion, flour, and pepper; toss to combine.
Preheat oven to 400°.
Divide parsnip mixture into 10 equal portions (about 1/4 cup per portion), shaping each into a 1/2-inch-thick patty.
Heat 1 tablespoon oil in a large nonstick skillet over medium heat. Add 5 patties; cook 2 minutes on each side or until golden brown. Place cooked patties on a baking sheet. Repeat procedure with remaining oil and uncooked patties. Bake latkes at 400° for 20 minutes, turning after 10 minutes. Sprinkle evenly with 1/4 teaspoon salt. Serve with horseradish cream.