Minted Beet Salad

Wrap the beets together tightly in foil in 2 batches and roast them in the middle of a preheated 350 °F. oven for 1 1/2 hours, or until they are tender. The beets may be roasted 2 days in advance and kept covered and chilled. Unwrap the beets carefully and let them cool until they can be handled. Slip the skins off the beets, cut off the stems, and cut the beets into 1/4-inch pieces. In a large bowl whisk together the vinegars, the sugar, and salt to taste, add the oil in a stream, whisking, and whisk the dressing until it is emulsified. Add the beets to the bowl with two thirds of the mint and toss the salad until it is combined well. Line 4 plates with the lettuce leaves, divide the beet mixture among them, and sprinkle each salad with some of the remaining mint.