Lightly coat a 10-inch (25 cm) nonstick ovenproof skillet with olive oil cooking spray. Add olive oil and place over medium-low heat.
Throw in the potatoes and onion. Cook, stirring frequently, until the potatoes are tender and the onion has turned golden brown.
Stir in the garlic, parsley, pepper, and salt (if desired). Continue to cook, stirring, for another 5 minutes.
Transfer potato mixture to a large bowl. Mix in egg substitute. Preheat the broiler.
Wipe up the skillet using paper towels and recoat the skillet with cooking spray. Add potato-egg mixture to the skillet and cook over low heat, pressing the potatoes down with a spatula so that they are completely covered by the egg.
Cook, covered, until the egg on the sides of skillet is slightly brown and egg mixture starts to solidify, about 10 minutes.
Place under the broiler until center of omelet is cooked through and the top is golden brown, about 3 to 5 minutes.
Carefully flip tortilla onto a large serving dish and cool to room temperature. Cut into very small wedges or 1 1/2-inch squares to pick up with a toothpick.