Cover potatoes with cold water by 1 inch; add cooking salt. Gently simmer 4 to 5 minutes until outsides soften but centers remain slightly firm. Drain; steam-dry 3 minutes.
Heat 1 tablespoon oil in a large 12-inch skillet over medium-high. Cook peppers and onion with half seasoning salt 7 to 8 minutes until edges soften; transfer to a bowl.
Add remaining oil and butter. Spread potatoes fairly evenly; cook undisturbed 5 minutes until undersides brown.
Turn in sections; cook 8 to 10 minutes more, occasionally turning, until browned on several sides and tender throughout. Lower heat if browning before centers soften.
Return peppers and onion with remaining salt and pepper; toss 2 to 3 minutes until hot. Serve immediately.