Bring the water to a boil and remove from the heat. Add sugar and dissolve then add the vinegar. Add the finely chopped mint and set aside to infuse for 1/2 to 1 hour.
This is of course a very traditional sauce for lamb. In England there is an older more traditional recipe using malt vinegar but I think that the malt vinegar just kills the fresh mint flavor and was used to hide the musty taste of older lamb (mutton). In the old days lamb was butchered older and had a more gamy flavor, not very fashionable today because of it's very heavy gamy flavor. Mint sauce of course was an excellent way of cutting that heavy flavor. Mint Sauce can be used on roasted leg of lamb or roasted racks of lamb.I still think that mint sauce has a place at the table. A lovely young roast leg of lamb is always good with a little freshly made Mint Sauce. Try and stay away from bottled mint sauces. As you can see it takes almost nothing to make it fresh and the results are far superior.