Grind fennel seeds in spice mill or mortar and pestle; set aside. Heat oil in a large skillet over medium. Add onion, season with salt, and cook, stirring often, until golden and soft, 810 minutes. Add chiles, garlic, coriander, and reserved fennel and cook, stirring, 1 minute. Add tomatoes, crushing with your hands as you go, and juices; reduce heat and gently simmer, stirring occasionally at first and more frequently as sauce thickens, until sauce is thickened and jammy, 5060 minutes. Let cool; discard chiles. Pulse sauce in a food processor to a coarse puree. While tomato sauce is cooking, rinse potatoes in a colander under cold water until water runs clear. Transfer to a large pot and pour in cold water to cover by 1″. Season with salt and bring to a gentle simmer; cook until potatoes are just tender, 2025 minutes. Drain and transfer potatoes to a kitchen towellined rimmed baking sheet. Chill (uncovered) until cold, about 1 hour. Meanwhile, whisk egg yolk, garlic, and vinegar in a small bowl to combine. Whisking constantly, gradually drizzle in grapeseed oil and continue to whisk until aioli is thickened and emulsified. Season with salt; stir in 1 tsp. water. Fit a large pot with thermometer and pour in vegetable oil to measure 3″. Heat over medium-high until thermometer registers 325°F. Working in batches, fry potatoes until skins begin to slightly bubble around edges but do not brown, about 2 minutes. Using a slotted spoon, transfer potatoes to paper towels to drain. Reheat oil until thermometer registers 375°F. Working in batches, refry potatoes until golden brown and very crisp, about 3 minutes per batch. Transfer to fresh paper towels to drain. Toss potatoes, paprika, and cumin in a large bowl; season with salt. Serve potatoes with tomato sauce and aioli. Tomato sauce can be made 4 days ahead; cover and chill. Bring to room temperature before serving. Potatoes can be blanched 1 day ahead. Let cool; cover and chill.