Begin by preheating your oven to 375°F (190°C). Allow the oven to fully heat before placing the casserole inside, as this helps the dish cook evenly and ensures the cheese melts properly while the sauce becomes creamy and bubbly.

Place the chicken breast on a clean cutting board and use a sharp knife to cut it into small, bite-sized cubes. Try to keep the pieces roughly the same size so they cook evenly. Cutting the chicken into smaller pieces also helps it blend well with the broccoli in every bite.

Set a large skillet on the stove over medium heat. Once the pan is warm, add the diced chicken. Cook the chicken for several minutes while stirring occasionally. As it cooks, use a spatula to move the pieces around the pan so they brown evenly. The goal is to lightly brown the chicken on the outside without fully cooking it yet, since it will continue cooking later in the oven.

While the chicken is cooking, rinse the broccoli under cold running water. Place it on the cutting board and cut it into small florets. Smaller pieces work best for casseroles because they cook more evenly and absorb the creamy sauce better.

In a medium mixing bowl, combine the cream cheese, heavy cream, minced garlic, onion powder, salt, and black pepper. Use a spoon or whisk to mix everything together until the mixture becomes smooth and creamy. If the cream cheese is very firm, letting it sit at room temperature for a few minutes will make it easier to mix.

Take a baking dish and add the lightly browned chicken along with the chopped broccoli. Spread the ingredients out evenly across the dish so the chicken and vegetables form a balanced base for the casserole.

Pour the prepared creamy sauce over the chicken and broccoli mixture. Use a spoon or spatula to gently stir everything together so the sauce coats the chicken and broccoli evenly.

Sprinkle the shredded cheese evenly across the top of the casserole. Try to distribute the cheese so it covers most of the surface, which will create a delicious golden layer once baked.

Place the baking dish into the preheated oven and bake for 25 to 30 minutes. During this time, the chicken will finish cooking, the broccoli will become tender, and the cheese will melt and turn lightly golden on top.

Once the casserole is fully cooked and bubbling around the edges, remove it carefully from the oven. Allow it to rest for about 5 minutes before serving. This short resting time helps the sauce thicken slightly and makes the casserole easier to portion and serve.