To a large sauté/frying pan add the oil butter over medium heat.
When the butter is melted and bubbling, add the mushrooms and cook until they start to brown about 10 minutes, stirring often. If they give off any liquid, cook until it evaporates.
Add the onions and cook for about 8 minutes until they start to soften. Stir in the garlic, sage, salt and pepper, cook for 2 minutes. Add the rice and cook stirring for 5 minutes (this toasts the rice and coats in fat to separate the grains).
Add the stock and stir. Bring to a simmer and cover. Cook for 15 minutes until the rice is tender. Allow to rest for 5 minutes then fluff with a fork.