Preheat the oven to 325 degrees . In a large, ovenproof nonstick skillet, heat 2 tablespoons olive oil over medium-low heat. Add the potatoes, onion and 1 teaspoon salt and cook, stirring, until brown, about 20 minutes.
Add the beaten eggs to the pan and cook over medium-low heat until the sides set, about 3 minutes. Transfer the pan to the oven and bake until firm, about 10 minutes. Run a rubber spatula around the sides of the skillet to loosen. Set aside to cool. Slice into 6 wedges.
Meanwhile, using a food processor, puree the roasted peppers, mayonnaise, garlic and remaining 2 tablespoons olive oil; season with salt and pepper. Spread a thin layer of aioli on each toast. Place a frittata wedge on each of 6 toasts; top with the remaining toasts.