Step 1: Prepare the Chicken and Broccoli

1. Cook the Chicken: If you are not using pre-cooked rotisserie chicken, start by cooking your chicken breasts. You can boil, bake, or pan-sear them until cooked through (internal temperature reaches 165°F or 74°C). Let the chicken rest for a few minutes, then shred it with two forks or chop it into bite-sized cubes. Set aside.
2. Blanch the Broccoli: Bring a large pot of salted water to a boil. Add the fresh broccoli florets and blanch for just 2-3 minutes. You are looking for a tender-crisp texture and a vibrant green color—do not overcook it to mush!
3. Shock the Broccoli: Immediately drain the broccoli and transfer it to an ice water bath. This process, called “shocking,” stops the cooking process instantly and preserves its bright color and crisp texture. Once cooled, drain it thoroughly and pat it dry with paper towels. This is a crucial step to prevent a watery casserole.

Step 2: Build the Flavor Base

1. Preheat and Prepare: Preheat your oven to 375°F (190°C). Grease a 9×13 inch casserole dish or another 3-quart baking dish with butter or cooking spray.
2. Sauté Aromatics: In a medium skillet over medium heat, melt the 2 tablespoons of butter. Add the finely chopped yellow onion and sauté for 5-7 minutes, until it becomes soft and translucent, but not browned. This step develops a sweet, savory base for your sauce.

Step 3: Create the Velvety Low-Carb Sauce

1. Combine Wet Ingredients: In a large mixing bowl, combine the softened cream cheese, mayonnaise, and sour cream. Use an electric hand mixer or a whisk to beat them together until the mixture is completely smooth and free of lumps. Starting with room temperature ingredients will make this much easier.
2. Add Flavor and Seasonings: Whisk in the sautéed onions, chicken broth, Dijon mustard, and fresh lemon juice. Continue to mix until everything is well incorporated.
3. Incorporate Dry Seasonings: Add the garlic powder, onion powder, salt, and pepper. Mix again. Taste the sauce and adjust seasonings if needed. It should be boldly flavored, as it will be coating the chicken and broccoli.
4. Thicken the Sauce (Optional but Recommended): If using, sprinkle the ¼ teaspoon of Xanthan Gum evenly over the surface of the sauce. Immediately whisk it in vigorously for about 30 seconds until the sauce has thickened slightly. Be careful not to add too much, as a little goes a long way.

Step 4: Assemble and Bake the Casserole

1. Combine Ingredients: To the large bowl with the sauce, add the cooked, shredded chicken and the dry, blanched broccoli florets. Use a spatula to gently fold everything together until the chicken and broccoli are evenly coated in the creamy sauce.
2. Transfer to Baking Dish: Pour the mixture into your prepared 9×13 inch casserole dish. Spread it out into an even layer.
3. Add the Cheese Topping: Sprinkle the 2 cups of freshly shredded sharp cheddar cheese evenly over the top of the casserole, covering it from edge to edge.
4. Bake to Perfection: Place the casserole in the preheated oven and bake for 25-30 minutes. The casserole is done when the sauce is hot and bubbly around the edges and the cheese on top is fully melted and turning golden brown.
5. Optional Broil for a Crispier Top: For a beautifully browned and slightly crispy cheese crust, you can switch the oven to the broil setting for the last 1-2 minutes of cooking. Watch it carefully, as the broiler can burn the cheese very quickly!
6. Rest Before Serving: Remove the casserole from the oven and let it rest for at least 10 minutes before serving. This allows the sauce to set up and thicken further, making it easier to serve.