Add chopped potatoes and salt to a medium pot, cover potatoes with water, and cook for 5 minutes until a fork can pierce potatoes slightly. Drain and set aside.
In a large skillet over medium heat, add oil and butter; once butter is melted, add onion and bell pepper, cook for 1 minute, then add potatoes and cook for 5 minutes, stirring constantly. If you don't, the potato's starch will stick to the bottom of the skillet.
Add the garlic, rosemary, Cajun seasoning, and black pepper, and cook for 2 minutes. Stir in the remaining butter and cut off the heat. Add the cheese, and cover until cheese melts, about 2-3 minutes. Serve immediately and garnish with salsa.
