To make sauce: Heat oil in a frying pan over medium heat. Add onion and garlic and cook, stirring, until soft, about 7 minutes. Add tomatoes, wine, tomato paste, chile flakes, parsley, sugar and 3/4 cup water, and stir well. Reduce heat to low and simmer, uncovered, for 15 minutes. Remove from heat and let cool slightly. Transfer to a blender and process to a smooth puree. Season with vinegar, salt and pepper to taste. Set aside.
To make tortilla: Insert the tip of a small knife into a potato and twist the knife to cut the potato into 1/4-inch-thick chips. You do not want perfect slices—you want them to be uneven so there will be space between them as they cook. Repeat with remaining potatoes.
In a large, nonstick frying pan, heat oil over medium heat. Add potato slices 1 at a time to prevent sticking. Cook slowly, lifting and turning potatoes occasionally, until they are tender but not brown, 8 to 10 minutes. The potatoes will remain separate rather than form a "cake." You may have to do this in 2 batches.
Meanwhile, crack eggs into a large bowl, season with salt and pepper, and beat with a fork until slightly foamy.
Using a slotted spoon, transfer potatoes to eggs, pressing down on potatoes so they are completely covered. Drain oil from pan, setting aside 3 tbsp. Discard remaining oil or reserve for another use.
In a large frying pan, heat 2 tbsp. of reserved oil over medium-high heat until very hot. Add potato mixture and spread evenly with a rubber spatula. As the edges set, loosen with a fork to allow the uncooked egg to flow underneath. Continue to cook until almost set and bottom is golden brown, about 6 to 10 minutes. Turn carefully onto a plate, add remaining reserved oil to the pan, and slide the tortilla, browned side up, into pan. Cook, shaking pan to prevent sticking, until cooked through, 4 to 6 minutes longer.
Remove from heat and slide onto a serving plate. Can be served hot or room temperature. Cut tortilla into wedges and spoon tomato sauce over top. Garnish with parsley.
Adapted and reprinted with permission from "Kitchen Gypsy," by Joanne Weir, copyright 2015, published by Oxmoor House. Photo by Thomas J. Story.