In a Dutch oven or large stockpot, add potatoes and 1 tablespoon salt. Add cold water to cover potatoes by 1 inch.
Over medium-high heat, bring to a boil and partially cover the pot. Cook until potatoes are partially tender when pierced by a paring knife, about 5 minutes. Drain well, tossing in a colander to remove excess water.
Meanwhile, in a large skillet over medium-high heat, add butter and olive oil and stir until melted. Add onion and cook until softened, about 5 minutes.
Add potatoes and press into an even layer. Cook without stirring until browned on the bottom, about 5 minutes. Using a metal spatula, turn the potatoes in batches without stirring. Continue cooking and turning as needed, without stirring, until browned on the other side, about 15 minutes longer total.
Sprinkle evenly with seasoned salt to taste (I like 1/2 teaspoon seasoned salt) and toss gently to combine. Serve immediately.
Notes
Potatoes: Leave the skins on for a rustic side dish.
Seasoned salt: I usually use Lawry’s, but you can make Homemade Seasoned Salt with common pantry spices if you prefer.