This versatile Italian omelet can be made with many different fillings.
Roast pepper over a gas flame or under a broiler until blackened. Place in a paper bag to cool. Peel and seed, then cut into 1/4-inch-wide strips.
Combine pepper strips, eggs, milk, most of the scallions, tarragon, salt, pepper, and half of the goat cheese. Set aside.
Heat butter and oil in a 10-inch ovenproof saute pan (preferably nonstick) over medium heat. Add potatoes and cook on both sides until tender, about 8 minutes.
Heat oven to 350 degrees. Pour egg mixture over potatoes and cook over medium-low heat, pulling back edges with a rubber spatula until whole frittata is partially cooked, about 8 minutes. Sprinkle remaining cheese over frittata and place in oven. Bake until completely set in center, about 25 minutes. Garnish with tarragon sprigs and remaining scallions. Serve hot, warm, or at room temperature.
Leftover frittata should be served at room temperature.