Open each bun and cut in half. Lay buns out on sheet pan. Cover sheet pan with plastic wrap and lay another sheet pan on top of the buns to flatten the buns.
Bring a large pot of water to poaching temperature. Add salt, crab boil and carefully add the shrimp. Cook until the shrimp are opaque, about 3 to 5 minutes. Drain and set aside until cool enough to handle, about 5 minutes. Once cooled, slice each shrimp in half.
In a large bowl, add the celery, mayonnaise, 1 tablespoon chives, 3/4 red bell pepper (reserve remaining for garnish), lemon juice, mustard and salt and pepper, to taste, and toss to combine. Add the shrimp and toss to combine.
Place the baby greens into the base of the buns. Top with the shrimp and garnish with chives and remaining red bell pepper.
