In a saucepan, over medium heat, combine the beer, water, onions and sausage. Season with salt and pepper. Bring to a boil, reduce to a simmer and cook, covered, for 25 minutes. Remove the sausage from the pan and cool. Slice the sausage into 1 -inch pieces. In a large saute pan, over medium heat, melt the butter. Stir in the flour and cook for 4 to 6 minutes for a light brown roux. Stir in the vinegar, sugar and the reserved beer and onion mixture. Season with salt and pepper. Bring the mixture to a boil, reduce to a simmer and continue to cook until the sauce coats the back of a spoon, about 15 minutes. Add the sliced sausage and continue to cook for 5 minutes. Garnish with parsley.