1. Peel and mince the shallot, or slice it very fine. Mix it in a medium-sized non-reactive bowl with the vinegar and a nice pinch of salt and a few turns of freshly-ground black pepper. If possible, let stand a couple of hours.
2. Stir in the olive oil. Slice the tomatoes then toss them in the dressing.
3. Let sit a few minutes, stirring a couple of times, then pile on a plate and serve.