Put 2 lb red potatoes in a large pot, cover with cold water by an inch, and add a big pinch of salt. Bring to a boil, then cook 4 to 5 minutes. A paring knife should meet slight resistance.

Drain in a colander and let the potatoes sit uncovered for 5 minutes, until they stop steaming and the surfaces look chalky.

Stir together 1 1/2 teaspoons paprika, 1 teaspoon garlic powder, 1 teaspoon onion powder, 1 teaspoon kosher salt, 1/2 teaspoon black pepper, and 1/4 teaspoon cayenne pepper in a small bowl.

Heat 3 tablespoons vegetable oil in a large cast iron skillet over medium-high until shimmering. Add the potatoes in a single layer, in two batches if needed, and cook undisturbed for 5 to 6 minutes.

Flip the potatoes, add 1 medium yellow onion and 1 tablespoon unsalted butter, and cook 6 to 8 minutes more, turning every couple of minutes, until the onion is soft and the potatoes are browned on most sides.

Sprinkle the seasoning blend over the potatoes and toss 30 to 60 seconds until evenly coated. Taste, adjust salt, and serve hot.