In a large, oven-safe skillet, fry bacon to your liking. Set bacon to the side on a large plate (with paper towels to soak up excess fat). Spoon & save the bacon fat and put it off to the side.
Turn the heat to medium-high. Add 1 tbsp of bacon fat. Add the Brussel sprouts to the pan and stir occasionally until sprouts are golden brown, around 8-12 minutes. Set aside (on the same plate as the bacon if it’s big enough).
In the same skillet, add 1 tbsp bacon fat. Add the chicken and cook thoroughly. Set aside with the rest of the cooked ingredients.
Turn the heat to low-medium and add the onions & salt. Caramelize onions and then add garlic and cook until golden. Don’t burn the garlic.
Add cream cheese and heavy cream. Stir frequently to let the cream cheese melt. If you want a more liquid base, you can add the chicken broth at this point. Keep stirring until it’s well incorporated.
Once melted, add the cooked ingredients: chicken, Brussel sprouts, bacon, and jalapeños (optional).
Stir well and make sure all ingredients are covered with the cream mixture. Sprinkle mozzarella cheese & parmesan cheese on top. Place skillet under the broiler for 3-4 minutes, until cheese is melted and starts to brown.