Place eggs in saucepan. Cover with 1 inch of water and bring to a rolling boil over high heat. Remove from heat; cover and let stand for 15 minutes.
Meanwhile, in a small bowl combine mayonnaise, relish, and mustard. Set aside. Heat a large skillet over medium-high heat. Cook ham 1 to 2 minutes per side until heated through.
Drain eggs; rinse in cold water. Peel under running water. Slice or coarsely chop eggs. Spread some of the mayonnaise mixture on toast. Top with ham, eggs, watercress, and any remaining mayonnaise mixture. Serve immediately.