For the shrimp stock
In a large stockpot, cook onions in butter on medium heat until tender, stirring frequently. Add flour and mix well. Gradually add shrimp stock, stirring until well-blended. Add potatoes and bring to a boil. Reduce heat to medium-low and simmer 20 minutes or until potatoes are tender. Stir in cheese and cook until melted, stirring frequently. Add shrimp, fish, and half and half. Salt and pepper to taste. Cook until heated through, stirring occasionally. Serve immediately. For the shrimp stock Combine all ingredients and bring to a boil, then simmer for 20 minutes. Strain into a bowl.