Preheat the oven to 375°F. Lay the bacon onto a sheet pan lined with parchment paper. Roast for 25-30 minutes, or until the bacon is crisp. Transfer the bacon to a plate lined with paper towels to drain.
Meanwhile, bring a medium pot of water to a boil. Gently place the eggs into the boiling water. Boil for 10 minutes. Transfer the eggs to a bowl of ice water. Let cool. Crack the eggs. Cut them into quarters lengthwise, and then into half crosswise.
Combine the mayonnaise, Dijon mustard, lemon zest, lemon juice, garlic, paprika, coriander, and cayenne pepper in a large mixing bowl. Whisk to combine. Fold in the eggs, celery, green onions, radishes, dill, and chives.
Roughly chop the bacon. Fold ¾ of it into the egg salad. Season with salt and black pepper to taste.
Distribute the watercress or arugula onto the toasted bread. Spoon a large mound of egg salad over top. Garnish with the remaining bacon and any extra herbs.
