Preheat your oven to 400°F (200°C). Grease a 9×13-inch baking dish and set it aside.

Bring a large pot of salted water to a boil. Cook the whole wheat pasta according to the package instructions until al dente. Drain and set aside.

In a large sauté pan, heat olive oil over medium heat. Add diced onion and cook for 4–5 minutes, until softened. Stir in the minced garlic and cook for another 1 minute, just until fragrant.

Sprinkle the flour over the onions and garlic. Stir constantly for 1 minute to cook off the raw flour taste. Slowly pour in the chicken broth while whisking to avoid lumps. Then stir in the milk and Dijon mustard. Let the sauce simmer for 4–5 minutes, stirring occasionally, until it thickens slightly. Season with salt and pepper to taste.

In a large mixing bowl (or in the pan if it’s big enough), combine the cooked pasta, chicken, broccoli, 1 cup of the shredded cheddar, and the sauce. Toss everything together until evenly coated.

Transfer the mixture to your prepared baking dish. Sprinkle the remaining 1/2 cup of cheddar cheese over the top. Bake uncovered for 15–20 minutes, until the cheese is melted and the casserole is bubbling around the edges.

Let it rest for a few minutes before serving. This gives the flavors time to settle and makes it easier to scoop.