Add 25g butter and a tsp of olive oil to a large sauté pan and sweat the onion, celery and garlic on a medium heat for 15 minutes until the onions are starting to turn transparent and golden. Add the mushrooms and continue to cook until the mushrooms let off their moisture and begin to fry – about 8 minutes. Add the chicken stock, bay leaf, half the tarragon and parsley and bring back to a boil, simmer for a few minutes to reduce slightly. Set aside and keep warm.