Sauté the Mushrooms Heat your oil in a saucepan over medium heat. Sauté the garlic until it’s just turning golden (about 2 minutes), then toss in the mushrooms with a pinch of salt. Cook them down until they are tender and have released their moisture (about 7 minutes).
Build the Roux Sprinkle the flour over the mushrooms and stir constantly. You want to cook the flour for about 30 seconds until it smells nutty. This prevents a "raw" flour taste and gives the gravy its body.
Simmer and Reduce Slowly whisk in the vegetable stock. Add the porcini powder, black pepper, and that fresh rosemary sprig. Bring it to a gentle simmer, then turn the heat to medium-low. Let it cook for 30 minutes. This slow simmer is where the magic happens—the flavors concentrate and the gravy thickens perfectly.
The Finishing Touches Stir in the tamari and nutritional yeast for that final hit of savory goodness. Remove the rosemary sprig, taste for salt, and let it rest for 10 minutes before serving. That rest is crucial for a silky-smooth finish!