Preheat the oven to 375°F (190°C). Place the chicken breasts on a baking sheet and bake for about 20-25 minutes until cooked through. Once done, shred the chicken with two forks.
In a saucepan, melt the butter over medium heat and add the sliced onions. Cook, stirring frequently, until the onions are golden brown and caramelized, about 15-20 minutes. Stir occasionally and adjust the heat if necessary to prevent burning.
Mix the shredded chicken with buffalo sauce until evenly coated. Set aside.
While the onions cook, toast the slider buns until lightly golden, either in a toaster or in the oven under the broiler for a few minutes. Keep an eye on them to avoid burning.
Assemble the sliders by layering the buffalo chicken on the toasted bun bottoms. Top with caramelized onions, sliced celery, and pickles for a fresh crunch and sweetness. Place the bun tops on to finish each slider.
Serve immediately while warm, with extra celery and pickles on the side if desired. Enjoy the spicy, savory richness of these flavorful sliders.