Preheat oven to 325 degrees. Heat 1 tablespoon oil in a 10-inch oven-safe skillet over medium-high heat. Add onion; saute, stirring, until golden, 8 to 10 minutes. Transfer to a small bowl.
Return the skillet to heat, and add 1 tablespoon oil. Add potatoes, cover, and saute until soft, stirring, 12 to 15 minutes. Transfer to the bowl with onions, and combine.
Whisk together eggs; add salt and pepper. Transfer to the bowl with onions and potatoes, and combine.
Return the skillet to heat, and add 1 tablespoon oil. Add egg mixture, and cook until edges set and start to brown, about 1 1/2 minutes. Cover, and bake in the oven until set, about 10 minutes. Remove cover, and broil until top is golden.
Meanwhile, slice bread in half crosswise, and reserve the top half for another use. Remove some of the inside from bottom half, and discard crumb. Toast bread under the broiler until golden. Remove, brush with 3 tablespoons oil, and rub with garlic clove. Toss frisee with remaining tablespoon oil, vinegar, and a pinch of salt and pepper. Scatter greens over bread, and slide torta over greens. Cut in wedges, and serve.