Heat oil in a medium pot over low-med heat. Add onions and sauté for 1 minute.
Add mushrooms, salt, pepper, sage and sauté until mushrooms are browned, roughly 8 minutes. Add red wine and vinegar and cook until liquid almost evaporates, roughly 30-60 seconds.
Stir in stock and bring to a boil until reduced, about 15 minutes.
Combine flour and water in a bowl and stir until smooth.
Whisk flour mixture into the pan and simmer on low for 2 minutes or until slightly thickened, stirring occasionally.
Optional: Take about half of the mushroom gravy and blend it for about 10 sec in a blender and add it back to the pot for a less chunky gravy.
Remove from heat. Stir in butter and allow to cool and thicken slightly before serving.