Put butter in a bowl (it'll soften while the eggs are cooking if it came right from the fridge).
Hard boil eggs.
While eggs are cooking, chop onion.
Once eggs are cooked, cool them in cold water for a few minutes then peel.
Using a fork, mash eggs, butter, and salt & pepper together until mixed.
Add onion and mix.
Spread mixture onto bread and enjoy!
Notes
If you're finding that the egg mixture isn't sticking together at all, add in a little more butter. You really don't need much butter for it to all stick together, though.
Making it with chives instead of onions is a tasty alternative.
I sometimes eat egg spread with a bowl of tomato soup. Easy weeknight dinner!