Heat oil in a small saucepan over medium heat. Add onion and sauté until soft, 6 minutes. Stir in mushrooms and herbs and season with salt and pepper. Cook until soft, about 5 minutes more. Add flour and cook for 1 minute.
Pour in 2 cups of vegetable stock and whisk to combine. Bring to simmer and cook for 5-10 minutes, until the flavors have melded and the mixture has thickened slightly. If the mixture is too thick, gradually add more vegetable stock.
Taste and season with more salt and pepper if needed, then serve warm