Peel potatoes and onion. Quarter each lengthwise, then cut crosswise into 1/4-inch-thick slices.
Heat a 10-inch ovenproof nonstick frying pan over high heat and add all but 1 teaspoon olive oil. When oil just starts to smoke, add potatoes and onion. Stir, turning vegetables to coat with oil. Reduce heat to medium-low and gently turn vegetables often until potatoes are tender when pierced, 15 to 20 minutes.
Preheat broiler. In a large bowl, beat eggs with salt and stir in potato mixture.
Drizzle remaining 1 teaspoon oil over bottom of frying pan (no need to wash) and return to medium-low heat. When pan is hot, add potato-egg mixture, spreading evenly. Cook until bottom is set and golden brown, 4 to 6 minutes.
Set pan under broiler about 4 inches from heat and broil, rotating pan as needed to cook evenly, until egg sets and lightly browns, about 3 minutes. Cover pan with a large plate; holding the two together tightly, invert. Lift pan off. Serve omelet warm or at room temperature, cut into wedges.
Note: Nutritional analysis is per serving.