Cover potato cubes with cold water by 1 inch, add cooking salt and bring to a gentle simmer. Cook for 4 to 5 minutes until the outside softens while the centers remain slightly firm; drain and steam dry for 3 minutes.
Heat 1 tablespoon oil in a large 12-inch skillet over medium-high heat. Cook peppers and onion with half the seasoning salt for 7 to 8 minutes until tender at the edges, then transfer to a bowl.
Add remaining oil and butter to the pan. Spread potatoes into a fairly even layer and cook without stirring for 5 minutes until the bottom faces brown.
Turn potatoes in sections and cook for 8 to 10 minutes more, turning occasionally, until browned on several sides and tender throughout. Lower the heat if the outside colors before the centers soften.
Return peppers and onion to the pan with remaining salt and black pepper. Toss for 2 to 3 minutes until hot, then serve immediately.