For the dressing, combine all the ingredients in a small bowl.
For the salad, cut around the stems of the tomatoes with a paring knife and reserve the stem "caps" to place back on the tomatoes. Cut a small slice from the base of the tomatoes so they will sit flat after stuffing. Cut each tomato horizontally into 5 slices, each about 1/3 inch thick. (They should be cut and arranged so that the slices can be reassembled later.)
Place a bottom slice from each tomato in the center of each of four salad plates. Cover each slice with some of the cucumber slices and a slice of onion and sprinkle with fleur de sel and a little dressing. Place the second consecutive slice from each tomato on top of the first. Cover with some of the feta slices and spoon about 1/2 teaspoon of dressing over the cheese. Add the third slice of tomato and cover with cucumber slices, onion, and a little more dressing. Add the fourth tomato slice and cover with feta cheese and a dash of dressing. Finish by adding the last tomato slice, with the hole from the missing stem. Reinsert the stems for a nice presentation.
Divide the olives and any remaining cheese or cucumber among the plates, scattering them around the tomatoes. Sprinkle the tomatoes with some fleur de sel and any extra dressing, garnish with oregano leaves, and serve cool but not cold.