Heat up the olive oil in a large pot and add half of the mushrooms. Cook until nicely browned, remove from the pot, and set aside.
To the same pot, add 2 tablespoons of butter, onion, garlic, and the remaining sliced mushrooms. Cook for about 5 minutes.
Add the rice to the mushrooms and cook, stirring continuously, for about 2-3 minutes.
Add about 1 tablespoon of vegan butter (optional), the reserved browned mushrooms, and the parsley.