1. In a large skillet over medium heat, add one tablespoon olive oil, swirl the pan to coat, and add the onion and the mushrooms. Sauté, stirring frequently, until the onion is soft and most of the mushroom liquid has cooked off, about 6-7 minutes. Add the garlic and sauté for another minute. Transfer to a bowl.
2. Return the skillet to the heat, add the remaining 4 tablespoons of olive oil and the rice flour. Use a whisk, stirring until the flour has browned lightly, 3-4 minutes. Slowly pour in about ½ cup of broth, whisking the entire time until smooth. Be careful as you pour—the broth may splatter. Continue adding broth slowly and whisking until all the broth has been added and the gravy is smooth. Add the coconut aminos, rosemary, and the mushroom mixture to the gravy. Reduce the heat to maintain a gentle simmer, and cook, stirring occasionally, for about 10 minutes.