1. Peel potatoes; cut into 2-inch pieces. Bring potatoes, 2 tsp. salt, and water to cover to a boil in a large Dutch oven over medium-high heat; boil 20 minutes or until tender. Drain. Return potatoes to Dutch oven, reduce heat to low, and cook, stirring occasionally, 3 to 5 minutes or until potatoes are dry.
2. Mash potatoes with a potato masher to desired consistency. Stir in warm buttermilk, warm milk, melted butter, pepper, and 1 tsp. salt, stirring just until blended.
3. Stir in Gruyère cheese, caramelized onions, and parsley, and spoon the mixture into a lightly greased 2 1/2-qt. baking dish or 8 (10-oz.) ramekins. Bake at 350° for 35 minutes.