Step 1

Preheat the oven to 400 degrees F.

Step 2

Cook the pasta in a large stockpot of generously-salted boiling water, until al dente, according to the package directions.

Step 3

About 1 minute before the pasta finishes cooking, add the broccoli to the boiling water and stir.

Step 4

Drain the pasta and the broccoli.

Step 5

Heat the butter in a large sauté pan over medium-high heat.

Step 6

Add the onion and sauté, stirring occasionally, until translucent, about 3 minutes.

Step 7

Add the mushrooms and the garlic and sauté, stirring occasionally, until the mushrooms are tender, about 5 minutes.

Step 8

Sprinkle the flour evenly over the onion mixture and stir until combined.

Step 9

Cook the onion mixture, stirring occasionally, until the raw flour smell disappears, about 1 minute.

Step 10

Add the stock and stir until most of the clumps are gone.

Step 11

Add the milk, the Dijon, the salt, and the pepper and stir until combined.

Step 12

Continue cooking the sauce until it reaches a simmer.

Step 13

Transfer the pan from the heat and stir in 1 cup of the shredded cheese until combined.

Step 14

Taste the onion mixture for seasoning.

Step 15

In a large 9x13-inch baking dish, toss the cooked pasta-broccoli mixture, the onion mixture, and the chicken together until combined.

Step 16

Smooth out the casserole into an even layer.

Step 17

Bake uncovered for 15 minutes.

Step 18

Sprinkle the remaining shredded cheese evenly on top of the casserole and bake until the cheese is melted, about 10 minutes.

Step 19

Serve.