Heat oil in a large nonstick skillet over medium heat. Add potatoes, onion, and salt. Use a spatula to coat potatoes with oil. When oil begins to bubble, reduce heat to medium-low and poach, turning frequently, until potatoes are tender but not browned, 20–25 minutes. Using a slotted spoon, transfer potatoes and onion to a large heatproof bowl. Add eggs and stir gently to combine; do not break up potatoes. Strain oil into a glass measuring cup; wipe out skillet. Heat 3 Tbsp. reserved oil from measuring cup in skillet over medium-high heat. Add egg-potato mixture and cook, stirring constantly but gently to keep potatoes intact, until eggs begin to set (eggs will look scrambled), about 2 minutes. Spread mixture in an even layer; reduce heat to
medium-low. Cook tortilla, shaking pan occasionally to prevent it from sticking, until eggs are nearly cooked through, about 12 minutes. Flip the tortilla with the help of a large plate or tortilla flipper, cook about 5 more minutes. Slide tortilla onto serving tray and let rest for 5 to 10 minutes. Cut tortilla into slices or squares and serve.