Cook The Pasta: Cook the 16-18 oz penne in water salted with 3 tbsp kosher salt (this flavors the pasta as it cooks, and most of the salt drains out afterward) until al dente. Drain and transfer to a large 9x13 casserole dish.
Make the taco seasoning (if homemade): Combine 1 tbsp chili powder, 2 tsp cumin, ½ tsp paprika, ½ tsp garlic powder, ½ tsp onion powder, ¼ tsp oregano, and ¼ tsp black pepper in a small bowl.
Sauté the vegetables: In a large skillet, heat 1 tbsp avocado oil over medium heat. Add the diced onion and bell peppers, and cook until softened, about 5 minutes. Add the minced garlic and cook for another minute.
Add Chicken & Seasonings: Stir in the 3 cups shredded chicken, 2 tbsp taco seasoning, and 2 tsp salt. Cook until warmed through, 3-4 minutes, then turn off the heat.
Mix The Sauce: Pour 2 cups sour cream and 2 cups salsa over the pasta and stir to combine. Add the chicken and vegetable mixture to the casserole dish and stir to combine.
Top and bake: Sprinkle the 2 cups shredded cheese evenly over the top. Bake at 350°F for 25 minutes, until bubbly and golden.