Start by caramelizing the onions. Heat 1 tablespoon of olive oil over medium heat and add in diced onions, letting the onion soften and turn a golden brown (about 5 minutes).
Add the brown sugar and balsamic vinegar to the pan and stir to coat the onions, letting them caramelize even further (about 2 minutes). Remove pan from heat.
Combine ground chicken, egg, breadcrumbs, parsley, garlic, salt and pepper in a large bowl. Add the caramelized onions and mix completely.
Form the chicken mixture into 12 equal size mini-patties. Grill the patties over medium high heat for 3 to 5 minutes per side, or bake on a lightly greased baking sheet in a 400 degree oven for 10 minutes, flipping over half way.
Top each chicken patty with 1/4 slice of cheese, followed by two small pieces of cooked bacon.
Spread mayo on bottom side. Place sliced California avocado then chicken patty. Top with top buns, separate sliders and serve.