In 8- or 9-inch/20 or 23 cm nonstick skillet, heat oil over medium to medium-high heat; add potatoes and fry, carefully flipping and moving with spatula and reducing heat if edges are browning, for about 5 minutes.

Meanwhile, in large bowl, beat together eggs, parsley, thyme (if using), salt and pepper; set aside.

Add onion to skillet and continue to fry until onion is light golden and potatoes are tender with golden edges, about 10 minutes. Draining well with slotted spoon, transfer potatoes and onion to egg mixture, stirring in gently; let stand for 3 to 5 minutes.

Pour off and reserve all but generous 1 tbsp/15 mL of the oil from pan; heat over medium-high heat until oil begins to haze. Add egg mixture, smoothing with spatula; cook until bottom is lightly browned and egg is lightly set through, reducing heat if bottom browns too quickly before egg sets, 3 to 5 minutes.

Turn out onto plate; add scant 1 tbsp/15 mL of the reserved oil to skillet; transfer tortilla, uncooked side down, to pan and cook until bottom is browned, 3 to 5 minutes. Slip onto serving plate and cut into wedges.

Makes 4 to 6 servings.