Season crabs, prawns, mussels, potatoes and corn with salt and pepper. Then placed them in the steamer and cook until all are well cooked. Prawns and mussels may take only shorter times to cook compare with crabs, potatoes and corn so you have to check on it.
Heat a skillet or Dutch oven to medium heat. Melt butter with corn starch stirring continuously. Slowly add the rest of the ingredients and simmer and stir for 10 minutes. Your sauce should be rich looking and slightly thickened. Seasoned well with salt and pepper.
When seafoods and the vegetables are already well cooked, arranged them on a plate and pour the sauce over it. Garnish with some chopped parsley.