Recipes for tortilla Espanola, including this one, call for a generous amount of oil -- but much of it is drained off and doesn't end up in the finished dish.
Tortilla: Toss zucchini with salt in a colander set over a bowl. Let drain at least 30 minutes, pressing on zucchini a few times to remove excess moisture.
Meanwhile, heat 1 cup oil in a 12-inch cast-iron skillet over high heat until shimmering. 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 to 35 minutes. Add zucchini and cook, stirring a few times, until tender, 12 to 15 minutes.
Transfer potato mixture to colander and let drain 5 minutes, gently stirring occasionally. Whisk together eggs, Parmesan, and basil in a large bowl, then stir in potato mixture.
Preheat oven to 425 degrees. Wipe skillet clean, making sure to remove any browned bits. Heat remaining 1/4 cup oil in skillet over high heat 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 to oven and bake until center is set and edges are golden brown, about 12 minutes. Let cool 5 minutes.
Run a paring knife around edge of tortilla to loosen, then invert onto a wire rack. Using a large spatula or a second wire rack, flip tortilla back over and let cool completely, about 1 hour, before cutting into wedges.
Romesco: Pulse almonds and garlic in a food processor until coarsely ground. Add peppers, oil, and vinegar; puree until smooth. Store in refrigerator, covered, until ready to serve, up to 1 day; dollop onto tortilla wedges.