Whisk all the sauce ingredients in a heavy-bottomed saucepan.
Bring to a simmer over medium heat, then reduce to low and cook for 15 minutes until thickened.
Cool slightly and reserve half for serving.
Cut the chicken breasts into 2-3 pieces each so they're similar in size to the thighs.
Season all the chicken with salt and pepper.
Heat the oil in a large skillet over medium-high heat.
Brown the chicken in batches, 2 minutes per side.
Transfer the browned chicken to a slow cooker.
Pour half the cooked BBQ sauce over the chicken.
Cook on high for 2 hours or until the chicken reaches an internal temperature of 165 F.
For the pickled onions, bring the vinegar, sugar, salt, and spices to a boil in a small saucepan.
Pour over the sliced onions in a bowl.
Cool to room temperature, then refrigerate for 30 minutes before removing the onions from the pickling liquid.
Shred the meat using two forks.
Lightly toast the slider buns.
Toss the arugula with the olive oil and a pinch of salt.
To assemble the sliders, spread BBQ sauce on the bottom bun and top it with the chicken mixture, pickled onions, and dressed arugula. Finish with a top bun.