Heat olive 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, stir to combine, and cook for 1 minute.
Gradually pour in 2 cups of vegetable stock and whisk to combine.
Bring to a simmer and cook for 5-10 minutes. This will help all the flavors to meld into the gravy and give it a rich flavor, and the gravy will also thicken.
If the mixture is too thick, gradually add the vegetable stock reserved.
Season to taste and serve warm.
*If making ahead of time and refrigerating until ready to use, you might need a bit more vegetable stock to smooth the gravy when heating.