Heat the butter in a pot over medium-low heat. Add onion. Once it starts to brown add the garlic and cook until fragrant.
Stir in the flour, nutritional yeast, and soy sauce. The gravy should form into a paste.
Slowly whisk in the broth. Add the sage, thyme, salt, and pepper. Bring the gravy to a simmer, whisking constantly until it thickens. If it gets too thick, gradually whisk in some more broth. Taste test for seasoning adjustments. Serve. Lasts up to 5 days refrigerated.
Notes
To reheat, add the gravy to a pot over medium heat and whisk in some broth to bring it back to a smooth consistency.