Melt the bacon grease or butter in a saucepan over medium-high heat. Sauté the onions until soft and just barely beginning to brown, 5-7 minutes. Add the minced garlic and cook for another minute.

Add the sliced mushrooms and fresh thyme to the saucepan. Cook until the mushrooms are soft and their liquid has evaporated, 5-7 minutes. Add the dry red wine and boil briefly until it's reduced by half.

Stir in the ground porcini mushrooms and concentrated tomato paste. Add the beef broth and kosher salt. Cover and simmer on low, stirring occasionally, for 10 minutes.

Add salt and pepper to taste. If you want a thicker sauce, dissolve cornstarch in a little water and whisk it continually into the gravy to prevent lumps. Simmer until the gravy is thickened, 1-2 minutes.

Stir in a pat of butter at the end for extra richness. If you prefer the gravy thinner, add a little more water or broth. For a creamier gravy, stir in a little heavy cream.