Hard-boil the eggs by placing in a large saucepan; add water until the eggs are covered.
Put the pan on the stove and bring to a boil over medium heat.
Once it reaches a rolling boil, remove pan from the heat, cover, and let stand for 20 minutes.
Run cool water over the eggs. Refrigerate if not using at once.
When the eggs are cool, peel the hard-boiled eggs and slice in half, lengthwise.
Pop out the yolks into a separate bowl.
Add mayonnaise and mustard (if using) to taste (mixture should stick together), and blend with a fork until smooth.
Using a small spoon, put the yolk mixture back in the hollows of the egg halves. Sprinkle with a dash of paprika.
Refrigerate until ready to serve. To send in a lunchbox, pack eggs in a plastic container.