How to Make It
Put oil, onion and potato in a heavy, 10-inch skillet; cook over medium heat, turning occasionally, until tender, 15 to 20 minutes.
Preheat broiler. Beat together eggs, water, salt and pepper; pour over potato mixture. Spread out entire mixture, then cook over low heat until bottom is lightly browned, about 10 minutes.
Broil tortilla in upper third of oven until set in center and golden on top, 6 to 8 minutes.
While tortilla is cooking, make red pepper sauce. Put red peppers in a food processor and process until smooth. With motor running, gradually pour oil and vinegar into red pepper puree. Season with pepper.
Cut tortilla into wedges; serve warm or room temperature with red pepper sauce on side.