Melt the butter (or bacon grease, lard or beef drippings) in a pan over medium-high heat and fry the onions until beginning to brown.
Add the garlic and cook another minute.
Add the mushrooms and cook 7-8 minutes or until golden and the liquid from the mushrooms has evaporated.
Add the ground porcini mushrooms and cook for another minute.
Add the butter and melt.
Add the flour, stir to combine and cook for about 3 minutes, stirring continually.
Add the beef broth, vinegar, thyme, sugar, salt and pepper.
Whisk continually until the gravy is thickened.
Cover and simmer on low, stirring occasionally, for 5-10 minutes.
Add salt and pepper to taste. If you prefer the gravy thinner add a little more water (or heavy cream if using).
