Patatas Bravas (Spanish Crispy Potatoes) with Spicy Tomato Sauce

Patatas bravas -- crisp potatoes covered with aioli and/or tomato sauce -- have secured their spot on nearly every Spanish tapas menu. These potatoes are not fried, but rather baked to a crisp after being parboiled.

The potato recipe is (heavily) adapted from Serious Eats and the spicy tomato sauce from The New Spanish Table, an amazing Spanish cookbook. Try to use duck fat, as the crispiness they produce is much more than that of olive oil, which still works reasonably well if needed. Also of note, I prefer red potatoes to the russet potatoes called for in the Serious Eats recipe, as they seem to disintegrate less during the high heat baking.)

Ingredients
For potatoes

• 2 pounds red potatoes (see note above), rinsed, peeled if desired, and cut into 2-inch chunks
• Kosher salt
• 1 tablespoons white vinegar
• 2-3 tablespoons duck fat (or olive oil)
• Freshly ground black pepper

For spicy tomato sauce

• 1 small white or yellow onion, minced
• 4 garlic cloves, chopped
• 3/4 teaspoon sweet smoked Spanish paprika
• 1/4 tsp ground cumin
• 1 pinch crushed red pepper flakes
• 1 15 oz can diced tomatoes in their liquid
• 2 tsp white vinegar
• 1 pinch sugar
• Salt and pepper to taste
• 4 teaspoons olive oil

Instructions

1. Prepare the potatoes. Preheat oven to 500°F. Place potatoes in a large pot and cover with cold water by 1 inch. Add 2 tablespoons salt and vinegar. Bring to a boil over high heat, reduce to a simmer, and cook until exteriors are tender, about 10 minutes. Potatoes should show slight resistance when poked with a paring knife or a cake tester. Drain potatoes and transfer to a large bowl.
2. Add duck fat to bowl with potatoes. Season with pepper and more salt to taste, then toss with a large metal spoon until exteriors are slightly bashed up (those tiny nooks and crannies make for an extra-crunchy exterior.) and coated in a thin layer of potato/fat paste. Spread potatoes on a heavy rimmed baking sheets.
3. Transfer baking sheets to oven and roast until bottoms of potatoes are crisp and golden brown, about 15-20 minutes total. Using a thin metal spatula, flip potatoes and roast until second side is golden brown, another 10-15 minutes. 
4. Prepare the spicy tomato sauce while the potatoes are roasting. Heat the olive oil in a heavy medium saucepan over low heat. Add the onion and garlic and sweat until softened, but not browned, 5 minutes. Add the paprika, cumin, and red pepper, and stir for 30 seconds. 
5. Add the tomatoes and 1/2 cup water and bring to a simmer. Partially cover and cook on low heat until thick, about 45 minutes, stirring occasionally. Add the vinegar and sugar, and season to taste with salt and pepper. Let cook, and then puree in a blender. 
6. Serve immediately, or sauce keeps up to a week in the fridge, but also freezes well for a few months in an airtight container.
7. To serve: plate the potatoes, and top with a generous amount of sauce (or aioli, if you've made that.)

Serves 4.