Directions

1. In a large skillet melt 1 tablespoon of the butter over medium heat. Add onions and sugar. Cook, stirring, 25 to 30 minutes or until onions are deep golden brown. Remove from heat; set aside.
2. Meanwhile, place red potatoes and enough water to cover in a Dutch oven; bring to boiling over medium-high heat. Reduce heat to medium; cook 20 minutes or until tender. Drain. Repeat with sweet potatoes.
3. Preheat oven to 350°F. Coat an oval 3-quart baking dish with nonstick cooking spray. Beat red potatoes at medium speed until mashed. Add sour cream, 3/4 cup of the milk and 2 tablespoons butter; beat until smooth. Stir in pepper and 1 teaspoon salt. Spoon into baking dish.
4. Beat sweet potatoes until mashed. Add remaining milk and butter; beat until smooth. Stir in remaining salt. Spread sweet potatoes over red potatoes. Top with half the onions; swirl gently with a knife. Top with remaining onion mixture.
5. Bake, covered, 25 to 30 minutes or until heated through. Serve in glass bowls to show the swirl effect, if desired.