Meanwhile, heat 1 cup of oil in a 12" ovenproof skillet (preferably cast-iron) over high. Add potatoes and onion; reduce heat to low. Cook, stirring occasionally and scraping up browned bits from bottom of pan, until potatoes are tender but not taking on any color 30-35 minutes.

Preheat oven to 425°F. Wipe skillet clean, making sure to remove any browned bits. Heat remaining 1/4 cup oil in skillet over high until shimmering. Add egg mixture and reduce heat to low. Cook, running a flexible spatula around sides occasionally to prevent eggs from sticking, until edges begin to set, about 10 minutes.

Transfer skillet to oven. Bake until center is set and edges are golden brown, about 12 minutes. Let cool 5 minutes. Run a knife around edges to loosen, then invert onto a wire rack. Using a large spatula, flip tortilla back over; let cool completely. To serve, cut tortilla into wedges and top with romesco sauce.