Place eggs in single layer in large saucepan. Add enough water to cover at least 1-inch above eggs. Cover saucepan and bring to a boil. Remove saucepan from heat. Let eggs stand, covered, in hot water 15 minutes. Drain; immediately run cold water over edges or place in ice water to completely cool.

Peel and chop eggs. Combine chopped eggs, celery, mayonnaise, shallot, tarragon, vinegar, Dijon mustard, lemon zest, salt, and pepper in medium bowl. Cover and refrigerate at least 2 hours before serving.

To assemble sandwiches, top 4 slices toasted bread with desired fixings. Add egg salad and top with additional desired fixings and remaining toasted bread slices. Serve sandwiches immediately or cover and refrigerate up to 2 hours.