How to Make It
Preheat the oven to 350°. Put the grapes on a large rimmed baking sheet and toss with the confectioners' sugar and 1 tablespoon of the olive oil. Bake for about 15 minutes, until the skins start to wrinkle. Meanwhile, spread the walnuts on a baking sheet and bake for about 8 minutes, until golden brown. Let cool, then coarsely chop.
In a bowl, whisk the mayonnaise with the lemon juice, vinegar, granulated sugar, mustard and the remaining 1 tablespoon of olive oil. Season with salt and pepper.
In a large bowl, toss the lettuce with the celery, walnuts and grapes. Add the dressing and toss well. Serve right away.