Preheat the oven to 350F. Butter a large gratin dish. Mix together the potatoes and onions and place them in the base of the gratin dish. Season with salt and pepper. Sprinkle with the saffron. Place the mussels in a large dutch oven with 1/2 cup of water and boil until they open. Strain, reserving the cooking liquid. Remove the mussels from the shells and keep warm. Pour the mussel cooking liquid through a fine cheesecloth over the potatoes and onions. Add the bouquet garni. Bake for 50 minutes. Place the hake fillets over the potatoes and onions. Return gratin dish to oven for 8 minutes. Remove gratin from oven and decorate with the cooked mussels, and chopped parsley. Serve in the gratin dish.
