In a large skillet, preferable stainless steel or enameled cast iron, bring the water and onion to a boil. Add the milk, salt, and pepper, and add the fish fillets. Reduce the heat and cook for 5 or 6 minutes. Turn off the heat and remove the fillets to a warm platter.

Over medium heat, reduce the poaching liquid by half. Meanwhile, prepare the sauce. In a small saucepan, over medium heat, melt the butter. Add the flour and cook for a minute, while whisking, to cook the flour. Whisk in half the cup of poaching liquid. The sauce will thicken immediately. Add the rest of the liquid and whisk well. Add green grapes and return the sauce to a boil. Adjust the seasoning and serve the sauce with Hot Green Beans and Artichokes over the fish. Garnish with Parsley