Mix together the shallot, salt and sherry vinegar. Pour onto the salmon and let marinate for at least 1 hour, preferably 3.

Drain off the marinade, and cut the salmon into very thin slices. (This step can be done in advance.)

For the dressing, mix the mustard and pepper in a bowl with the red wine vinegar. Add the oil, whisking well.

Pour the dressing over the spinach in a large bowl, tossing gently. Place the salmon slices all around. Serve with buttered whole-wheat toast.