For the dressing, whisk the mustard, vinegar and garlic together in a small bowl, then very gradually whisk in the oil to make a thick, emulsified dressing. Season to taste with salt and pepper and set to one side.
Meanwhile, overlap the tomato slices on a large, oval platter and sprinkle with a few sea salt flakes and ground black pepper. Scatter over the shallots and the capers.
Whisk 2 tablespoons of warm water into the dressing to loosen it a little, then drizzle over the salad.