Heat Fat. To start, add your fat (butter, drippings, or oil) to a large skillet over medium heat.
Brown Mushrooms. Add sliced or minced mushrooms and the onion or shallot. Cook 7-10 minutes over medium heat, stirring regularly, until mushrooms are browned and the onion is softened. Add garlic and cook 1 minute more.
Add Flour (if using). Sprinkle mushroom mixture with flour (if using) and cook 1 minute to toast. (This removes the raw taste from the flour!)
Build Flavor. Next, add broth and 1 Tablespoon tamari, along with the salt, pepper, sage, and rosemary. Stir to scrape up any browned bits from the bottom of the pan!
Thicken. Stir to combine and whisk or stir constantly over medium-low heat until the gravy thickens. This takes 5-10 minutes at a gentle simmer, depending on what thickener you use.
Taste & Adjust seasonings as desired, adding more tamari, salt, pepper, or herbs, to taste. Serve warm over mashed potatoes, beef, etc. and store leftover gravy in an airtight container in the refrigerator 3-4 days.