Hard‑boil the eggs: Place eggs in a medium saucepan, cover with cold water, and bring to a gentle boil. Cook for 9–10 minutes, then drain and transfer to an ice bath until cool. Peel and chop finely.
Prepare the cottage cheese base: In a mixing bowl, combine the cottage cheese, Dijon mustard, lemon juice, a pinch of salt, and a few grinds of black pepper. For a smoother texture, blend briefly with a hand blender or whisk by hand until almost uniform but still slightly textured.
Mix in aromatics: Stir the chopped eggs, celery, onion, and dill or chives into the cottage cheese mixture. Taste and adjust seasoning with more salt, pepper, or lemon juice as needed.
Toast the bread: Lightly toast 4 slices of bread in a toaster, oven, or skillet with a little butter or olive oil until golden and crisp.
Assemble the sandwich: Spread a generous layer of the cottage cheese egg salad on one side of each toasted slice. Add a few leaves of lettuce, spinach, or arugula and a few slices of tomato or cucumber if desired, then close the sandwich and press gently.
Serve: Slice in half if you like and enjoy immediately. Store leftover cottage cheese egg salad in an airtight container in the fridge for up to 3–4 days.