Preheat your oven to 350°F. Line a 9x13 inch baking dish with parchment paper and lightly spray with non-stick cooking spray. Arrange the slider bun bottoms in a single layer across the dish, fitting them snugly together. This creates a foundation that keeps everything organized and makes assembly much faster than building individual sliders.

In a bowl, combine the shredded chicken with 1/2 cup of the BBQ sauce, stirring until evenly coated. Divide this mixture among the bun bottoms in the dish, spreading it in an even layer. Drizzle the remaining 3/4 cup BBQ sauce over the chicken layer, then evenly distribute the mozzarella and gouda cheeses on top. Scatter the thinly sliced red onion over the cheese, then sprinkle with the cilantro and black pepper. I like to add the onion raw because it keeps a nice crisp bite that contrasts beautifully with the soft, melted cheese.

Place the slider bun tops over the filling, then cover the entire dish tightly with aluminum foil. This traps steam and helps the cheese melt evenly while keeping the buns soft and moist. Bake for 15 minutes until the cheese begins to melt and the filling is heated through.

While the sliders bake, whisk together the melted butter, finely chopped cilantro, garlic powder, and a pinch of flaky sea salt in a small bowl. After the 15 minutes are up, carefully remove the foil and brush this mixture generously over the tops of each bun. The herb butter adds richness and a final layer of flavor that brings everything together. Bake uncovered for another 10 minutes until the buns are golden brown and the topping is fragrant.