Boil water with plenty of salt for the spaghetti.
Over medium-high heat, in a large frying pan, heat oil and butter. Sauté onion for 2-3 minutes, until opaque. Add minced garlic, and cook for about one minute, stirring constantly, being careful not to burn it. Mix in mushrooms. (It may seem like a lot in the pan, but the mushrooms will reduce eventually. Also if it seems like the oil and butter are absorbed too quickly by the mushrooms, don’t worry, add another tablespoon or so of olive oil). Cook 3-5 minutes, adding a pinch of salt in the last stages.
Add half of the sage, increase to high heat and add chicken stock. Bring to a boil, then reduce heat and simmer for 8 minutes.