Bring potato, 1 teaspoon salt, and water to cover to a boil in a Dutch oven; cook 20 to 25 minutes or until tender. Drain and keep warm.
Melt 2 tablespoons butter with oil in a skillet over medium heat; add onion, and cook, stirring often, 12 to 17 minutes or until tender. Add garlic, and cook 3 minutes. Stir in rosemary; remove from heat.
Mash potato with a potato masher; stir in 1/2 cup butter, half-and-half, and cheese until blended. Stir in onion mixture, remaining 3/4 teaspoon salt, and pepper. Spoon enough mixture into a decorative, ovenproof dish or 13- x 9-inch baking dish to fill bottom; pipe or dollop remaining mixture over top.
Broil, 3 inches from heat, 5 minutes or until top is lightly browned. Garnish, if desired.