In a medium saucepan over medium heat, melt 2 tablespoons of butter and 1 tablespoon of olive oil. Sauté finely chopped yellow onion and minced garlic for 3–5 minutes until soft and fragrant. Gradually whisk in 1/4 cup of all-purpose flour, then slowly pour in 2 cups of lower-sodium chicken broth and 1 cup of whole milk. Bring to a simmer (about 5 minutes), stirring constantly until thickened. Stir in seasonings and half of the shredded cheddar cheese until melted.

Preheat your oven to 375°F (190°C). While the oven warms up, cut 1 pound of boneless skinless chicken breasts into bite-sized pieces, seasoning them generously with kosher salt and black pepper. In a greased 9x13-inch baking dish, spread out the uncooked quinoa evenly as the base for your Cheesy Chicken and Broccoli Quinoa Bake.

Add the seasoned chicken pieces and 2 cups of broccoli florets to the baking dish over the quinoa. Pour the melted cheese sauce over the entire mixture, ensuring everything is well-coated. Stir gently but thoroughly to distribute the sauce evenly across the chicken, quinoa, and broccoli, creating a harmonious blend of flavors.

Cover the baking dish with aluminum foil and place it in the preheated oven. Bake for 45 minutes, removing the foil halfway through to allow gentle steaming and prevent excess moisture. Check for doneness at the 45-minute mark; the quinoa should be tender, and the chicken thoroughly cooked. If needed, bake an additional 5-10 minutes until everything is perfectly cooked.

Once fully baked, remove the casserole from the oven and give it a good stir. Sprinkle the remaining cheese on top for that delicious, melty layer. Return it to the oven and broil for 3-5 minutes until the cheese is bubbly and golden brown, creating a beautiful finish for your Cheesy Chicken and Broccoli Quinoa Bake.