Preheat your oven to 375°F, and arrange 4 individual gratin dishes on a baking sheet.
In a medium saucepan, combine the seafood stock, heavy cream, wine, tomato puree, and saffron.
Bring it to a gentle boil.
Add the shrimp to the boiling mixture, and cook for about 3 minutes.
Remove the shrimp and add the cubed halibut for another 3 minutes, then remove it.
In each gratin dish, layer shrimp and halibut, then sprinkle on Gruyère and Parmesan cheese.
Mix Ritz cracker crumbs, melted butter, Worcestershire sauce, and garlic powder.
Spread this mixture evenly over each dish and bake for 20 minutes or until golden on top.