Boil the eggs: Place eggs in a saucepan, cover with cold water by an inch, and bring to a gentle boil. Once boiling, cover, turn off the heat, and let sit for 10–11 minutes.
Cool and peel: Transfer eggs to an ice bath for 5–10 minutes. Peel under running water to make it easier.
Chop the eggs: Pat dry and chop into small chunks.
You want texture, not a paste.
Make the dressing: In a bowl, mix mayonnaise, Dijon, lemon juice, a pinch of salt, and pepper. Taste and adjust. It should be tangy and lightly salty.
Fold in add-ins: Stir in celery, red onion, and herbs.
Add the chopped eggs and gently fold to coat. Chill while you prep the bread.
Prep the toast boats: Using a small serrated knife, cut a rectangle inside each bread slice, leaving a 1/2-inch border. Gently press or remove some of the inner crumb to create a shallow “boat.” Save the soft crumbs for breadcrumbs later.
Toast the boats: Brush both sides and the inner walls with melted butter.
Toast in a skillet over medium heat 2–3 minutes per side until golden, or bake at 375°F (190°C) for 8–10 minutes, flipping once.
Fill and finish: Spoon the egg salad into each boat, mounding slightly. Sprinkle with paprika or more herbs. Add optional toppings.
Serve warm: These are best right away when the toast is crisp and the salad is cool and creamy.