1. To drain, press tofu to remove excess liquid. Slice tofu in half horizontally into two 1-inch-thick pieces. Invert a plate onto a jelly-roll pan and cover with plastic wrap. Arrange tofu pieces side by side in the center of the plate. Cover tofu with plastic wrap, then top with another dinner plate. Weigh the plate down with a can. Let stand for 30 minutes, until tofu is firm and most of the water has drained out.
2. Combine tofu, celery, mayonnaise, onion, lemon juice, mustard, turmeric and salt in a bowl; mash tofu mixture with a fork until smooth. Cover and refrigerate, 1 hour, for flavors to blend.
3. Spread tofu mixture on 4 slices of bread; top with watercress and remaining 4 slices of bread.