Boil the eggs: Place eggs in a pot, cover with cold water by an inch, and bring to a rolling boil. Turn off heat, cover, and let sit for 10–12 minutes.
Chill and peel: Transfer eggs to an ice bath for 5 minutes, then peel and pat dry.
Make the dressing: In a bowl, whisk mayonnaise, Dijon, lemon juice, salt, and pepper until smooth.
Chop the mix-ins: Finely chop eggs. Stir in onion (or shallot), herbs, and celery if using.
Fold in the dressing until creamy. Taste and adjust seasoning.
Prep the avocados: Halve avocados and remove pits. Scoop out a tablespoon or two from each center to make a deeper well.
Mash the scooped avocado into the egg salad for extra creaminess, or save for another use.
Stuff and garnish: Spoon egg salad into avocado halves. Finish with paprika, chives, bacon bits, capers, or everything seasoning.
Serve: Enjoy immediately, or chill for 10 minutes for a firmer texture.