Boil potatoes until barely tender, 8 to 10 minutes. Drain and cool under cold, running water. Cut potatoes into 1/2-inch pieces. Heat a cast iron skillet. Add oil and butter. Add onion and saute until tender. Add potatoes and season with salt, pepper, cayenne and garlic salt. Cook until lightly browned and crisp. In a separate pan prepare eggs as desired (scramble, fry or poach). Sprinkle eggs with cheese and cover pan to melt. Serve potatoes topped with eggs, salsa and sour cream