Preheat the oven to 200 degrees F. Wrap the bread slices in foil and place them in the oven to warm. Meanwhile, heat the butter in a non-stick skillet set over medium heat. Add the onion and cook until tender, about 3 to 4 minutes. Add the slices of corned beef and the water, cover, and cook until the corned beef is just heated through, about 2 or 3 minutes. Remove the bread from the foil and spread Dijon mustard on one side of each slice. Divide and mound the warmed corned beef and onion on top of 2 of the slices. Top with the other 2 bread slices, mustard-side down. Cut each sandwich in half and serve immediately.