Scrub the Yukon gold potatoes under cold running water and leave the skins on. Cut into ¾-inch (2 cm) cubes, keeping them as even as possible.
Dice the yellow onion and red bell pepper into ½-inch (1.25 cm) pieces. In a small bowl, stir together the smoked paprika, garlic powder, black pepper, and ½ teaspoon kosher salt.
Place the potato cubes in a 4-quart saucepan and cover with about 2 quarts (2 L) cold water, so the water sits about 1 inch above the potatoes. Add 1 tablespoon kosher salt and stir once.
Bring the water to a boil over high heat, about 8 minutes. Reduce heat to medium-high and cook for 5 to 6 minutes, until a knife meets slight resistance in the center and the edges look just a little frayed.
Drain the potatoes in a colander and shake it firmly a few times to rough up the surfaces. Spread in a single layer on a rimmed baking sheet or wire rack and let sit for 3 to 5 minutes until dry and matte.
Set a 12-inch cast iron skillet over medium-high heat for about 2 minutes. Add 2 tablespoons olive oil and swirl to coat. Wait until the oil shimmers and flows like water, about 1 minute.
Add the potatoes to the skillet in a single layer, shaking gently to spread them out. Cook without stirring for 6 to 7 minutes, until the bottoms are golden brown.
Flip the cubes with a thin metal spatula and cook for about 5 minutes more, turning occasionally, until most sides have color.
Reduce heat to medium. Add 2 tablespoons butter, the diced onion, and diced red bell pepper. Sprinkle the spice mix over everything and toss to coat.
Cook for 6 to 8 minutes, stirring every minute or two, until the onion is soft and translucent and the potatoes are crisp and deep golden on most sides.
Taste a potato cube and add a pinch more salt if needed. Sprinkle with chopped fresh parsley if using, and serve immediately while the crust is at its crispest.