How to Make It
Combine water, vinegar, and onion in a medium saucepan; bring to a boil over high. Squeeze juice from lemon halves into pan, and add lemon halves. Reduce heat to medium, and simmer 10 minutes. Remove from heat; discard solids. Add scallops to pan, cover, and let stand 2 minutes. Using a slotted spoon, remove scallops to a plate; discard cooking liquid. Place scallops in refrigerator, uncovered, and allow to cool completely, about 15 minutes.
Stir together mayonnaise and spicy chili crisp in a small bowl. Combine scallops, celery, herbs, zest, and 3 tablespoons of the mayonnaise mixture in a large bowl. (Reserve remaining mayonnaise mixture.) Cover and chill until ready to serve.
Preheat broiler to high. Brush melted butter on top and bottom halves of split slider buns. Place buns on a baking sheet, and toast in preheated oven until slightly browned, 1 to 2 minutes.
Spread remaining mayonnaise mixture on toasted buns. Top evenly with scallop mixture, cucumber slices, and scallion slices. Cover with bun tops, and serve immediately.