In a large heavy pan, heat olive oil on medium heat. Add onions and cook/stir until softened and almost caramelized, 5-10 min. Add butter, garlic powder, black pepper, and salt. Stir well to combine. Add rice, stirring to coat. Add mushrooms and beef broth; stir. Bring to boil, cover with tight-fitting lid, and immediately lower to simmer.
Simmer 30 minutes, without opening lid. With lid still on, turn heat off and let sit 5 minutes. Fluff with fork and season with additional salt/pepper to taste. If desired, garnish with chopped chives or parsley.