Rinse tomatoes. Cut 1/2 inch off the top of each and reserve. With a spoon, carefully scoop out seeds and inner flesh, leaving a shell 1/4 to 1/2 inch thick; reserve interiors for another use. Set tomato shells and lids, cut side down, on a paper towel.
In a large bowl, mix mayonnaise, vinegar, and minced tarragon until smooth. Add crab, peas celery, and shallots; mix gently to coat. Add salt and peper to taste.
Line each of two salad plates with a lettuce leaf. If tomatoes do not sit flat, trim bottoms slightly as needed. Mound tarragon crab salad in tomato cups and set on lettuce leaves. Garnish with tomato lids and tarragon springs if desired.