Place the eggs in a saucepan and cover them with cold water. Bring the water to a boil over high heat. Once boiling, cover the saucepan, remove it from the heat, and let the eggs sit in the hot water for 10-12 minutes.
While the eggs are cooking, prepare a bowl of ice water. Once the eggs are done, transfer them to the ice water bath and let them cool for a few minutes.
Once the eggs are cooled, peel them and chop them finely.
In a mixing bowl, combine the chopped eggs, mayonnaise, Dijon mustard, chopped fresh dill (if using), chopped fresh chives (if using), salt, and black pepper. Mix until well combined.
Taste the egg salad and adjust the seasoning, if necessary.
To assemble the sandwiches, place a lettuce leaf on the bottom half of each croissant. Spoon a generous amount of egg salad on top of the lettuce.
If desired, add sliced tomatoes and sliced avocado on top of the egg salad.
Place the top half of each croissant on top of the filling to create a sandwich.
Serve the Egg Salad Croissant Sandwiches immediately, and enjoy!