In a large bowl combine Fiesta Seasoning Oil ingredients, stirring well to combine.
Place potatoes into a large pot of lukewarm water. Add salt. Bring to a simmer over high heat. Cook until potatoes are softened, but not fully cooked, 10 to 12 minutes total. Drain potatoes through colander, shaking a few times to release steam and soften edges. Allow to rest 5 minutes.
Gently toss boiled potatoes in bowl with Fiesta Seasoning Oil coating thoroughly. Divide potatoes evenly between two rimmed baking sheets. Cook, tossing potatoes once, and rotating baking sheets; until potatoes are crispy on the outside and tender on the inside, 25 to 30 minutes. Meanwhile make Nacho Cheese sauce.
Nacho Cheese Sauce:
In a large saucepan over medium, heat oil. Add onion, bell pepper, garlic, cumin, salt, and black pepper. Cook, stirring occasionally until onion has softened about 5 to 7 minutes. Slowly whisk in the milk, tomato paste, and vinegar, until well combined; allow to steep for 20 minutes.
In a large heatproof bowl, combine grated cheeses, Place a fine mesh strainer over the bowl containing the cheese. Strain milk mixture into the bowl. Discard solids from strainer.
Using a wooden spoon stir cheese mixture until smooth and saucy. If cheese doesn’t melt completely, set bowl over a pot of simmering water and stir until cheese melts and mixture comes together.
Top Copycat Taco Bell Cheesy Fiesta Potatoes with 3 cups Nacho Cheese Sauce, Sour Cream, tomatoes and pico de gallo if desired. Serve immediately.