Spicy Tomato Sauce: Heat oil in a small saucepan. Add onion and garlic; cook over medium heat for 10 minutes. Stir in tomatoes and red pepper; cover and cook for 20 minutes more. Let cool slightly, then puree in a blender or food processor; transfer to a serving bowl.
Tortilla: Heat oil in a 7- or 8-inch skillet. Add onions and cook over low heat for 10 minutes; add potatoes to skillet.
Season eggs with garlic salt and pepper; stir in cheese and add to skillet. Let cook for a few minutes, then carefully lift edges and tilt skillet so uncooked egg can run underneath. When eggs are almost set, carefully slide onto a plate. Cover with skillet and invert eggs back into skillet. Cook for a few minutes more or until cooked through.
Sprinkle with parsley and cut into wedges. Serve warm or at room temperature.
This dish is also great when wedges are topped with small spoonfuls of Spicy Tomato Sauce.