Heat both oils in a large oven-proof skillet over medium heat.
Add potatoes and salt. Cook potatoes in oil until tender but not browned 20-25 min. Remove potatoes from the oil and set aside.
Using the same oil cook the chopped onion until translucent, remove from skillet and transfer to container with potatoes.
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.
Preheat broiler to high. Cook tortilla, shaking pan occasionally to prevent it from sticking, until eggs are nearly cooked through, about 12 minutes.
Transfer skillet to oven and broil until top tortilla is just cooked, about 2 minutes.
Remove from oven. Invert a large plate over skillet. Using oven mitts (skillet and potatoes will be very hot; use caution), hold plate firmly over skillet and flip, releasing tortilla onto plate.
Let sit at room temperature for at least 20 minutes and up to 2 hours before serving.