Preheat oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish and set aside.

Pat diced chicken dry with paper towels. In a mixing bowl, toss chicken with olive oil, 1/2 tsp salt, 1/4 tsp black pepper, and 1/2 tsp garlic powder.

Heat a large skillet or Dutch oven over medium-high heat. Add the seasoned chicken and cook for 5-7 minutes, stirring occasionally, until browned on all sides and mostly cooked through. Remove chicken from skillet and set aside.

In the same skillet (no need to clean), add diced onion and cook for 3-4 minutes until softened. Add minced garlic and cook for another minute until fragrant.

Stir in the broccoli florets. If using fresh, cook for 3-5 minutes until tender-crisp. If using frozen, cook for 5-7 minutes until heated through and slightly softened. Remove vegetables from the skillet and set aside with the chicken.

Reduce heat to medium-low. Add the cubed softened cream cheese to the skillet and stir until mostly melted. Gradually whisk in the chicken broth and heavy cream until the sauce is smooth. Stir in the Dijon mustard.

If using, sprinkle Xanthan gum over the sauce and whisk vigorously for 30 seconds to thicken. Be careful not to add too much, a little goes a long way.

Stir in 3/4 cup of the shredded sharp cheddar cheese and all of the Parmesan cheese until melted and smooth. Season the sauce with salt and black pepper to taste, starting with 1/4 tsp each and adjusting as needed.

Return the cooked chicken and sautéed vegetables to the skillet with the creamy sauce. Stir gently to combine everything thoroughly, ensuring the chicken and broccoli are well coated.

Pour the mixture into the prepared 9x13 inch baking dish, spreading it evenly. Sprinkle the remaining 1/4 cup shredded sharp cheddar cheese over the top.

Bake for 20-25 minutes, or until the casserole is bubbly around the edges and the cheese on top is melted and lightly golden.

Remove from oven and let stand for 5 minutes before serving. Garnish with fresh chopped parsley.