Cut potatoes lengthwise in half; cut each piece lengthwise in half again. Cut each wedge crosswise into into 1/4-inch-thick slices. Place in large saucepan; add enough cold water to cover. Cover; bring to boiling. Stir in salt and onion; cook until potatoes are just knife-tender, about 5 minutes. Drain potatoes and onion.
Heat 10-inch cast-iron skillet over high heat. After 2 minutes, add butter; heat until melted and sizzling. Add potato-onion mixture; cook over high heat 2 minutes. Gently flip mixture over, trying not to mash potato pieces. Season with 1/2 teaspoon garlic salt, 1/8 teaspoon pepper and 1/8 teaspoon thyme.
Cook 5 minutes, turning potatoes over, seasoning with remaining garlic salt, pepper and thyme, until potatoes are browned and pieces are crispy. Serve warm.