Heat the oil in a large cast-iron pan over medium-high heat. When the oil is hot spread the potatoes out in a single layer in the pan and press down with a spatula. Let the potatoes cook for about 3 minutes then stir the potatoes and cook, stirring occasionally, for about 5 more minutes. Transfer the potatoes to a plate and set aside.
Melt the butter in the pan with the potatoes. Add the mushrooms, onions, celery, and bell pepper and cook, stirring occasionally, until mushrooms are well browned and vegetables have softened, about 10 minutes.
Stir in the paprika, garlic powder, thyme, oregano, cayenne, 1/2 tsp salt, and 1/4 tsp pepper. Cook, stirring often, while scraping the bottom of the pan to release any stuck on bits, about 5 more minutes.
Taste and season with additional salt & pepper if necessary.