Step 1Combine the white wine vinegar, shallots, Dijon mustard, salt and pepper in a small bowl. Whisk in olive oil in a thin stream.
Step 2Set aside the vinaigrette until you are ready to toss together the salad.
Step 3Clean the frisee under running water and remove any tough stems. Pat dry with paper towels.
Step 4Just before packing up your picnic, toss the tomatoes with the frisee. Toss with enough vinaigrette to coat.