Place a large pan on the stove, turn the heat to medium.
Add olive oil and butter into the pan, once the butter is melted, add the onions to the pan and stir until softened and translucent.
Add in the shallot and garlic. Stir and cook for 5-8 minutes.
Add in celery and cook for 15 more minutes.
Add in mushrooms, cook for 10 minutes until their juice is released.
Add in dried tarragon, salt and pepper. Stir well.
Add croutons and mix well. Pour the vegetable stock slowly on the stuffing and mix.
Cover and cook for 20 minutes. Stir occasionally.
Mix in shredded gruyer cheese and once it's fully combined, transfer to your serving dish.
Serve warm with extra shredded gruyer.