- First, preheat the oven to 350°. Grease an 8 x 11 casserole dish and set it aside. 
- Then, bring a large pot of salted water to a boil. Add the pasta and cook it according to the package instructions.  
- Meanwhile, while the pasta is cooking, heat the butter and olive oil in a large skillet over medium heat. Once the pan is hot, add the chicken pieces. 
- Cook the chicken until browned on both sides and cooked through. Add the onions and stir. Cook until the onions have softened. 
- Meanwhile, while the chicken and pasta are cooking, stir together the cream cheese, BBQ sauce, mayonnaise, and ¼ cup of the shredded mozzarella in a large mixing bowl. 
- Then, add the warm cooked pasta to the bowl with the BBQ sauce mixture. Stir it well to coat the noodles. 
- Next, add the cooked chicken and onions to the bowl with the sauce and pasta. Stir it all well to combine. 
- Then, spread the mixture evenly into the prepared baking dish. Cover it with the remaining ¾ cup of shredded mozzarella. 
- Bake in the preheated oven for 15-20 minutes until the cheese is melted and bubbling. 
- Serve warm.