Heat a chef's pan until the rim of the pan is hot to the touch.
Melt 1 oz. butter in pan and add the mushrooms and shallots. Cook until mushrooms have just released their moisture, but have not browned.
Add the halibut, peas and mushroom stock and bring to a simmer.
Simmer over low heat for 6 - 8 minutes or until fish begins to flake. Remove fish from pan and whisk remaining butter into broth. Remove pan from heat and add mint. Season to taste.
Place one piece of fish in the center of the plate and spoon 1/4 of sauce mixture on top and serve.