Heat the oven to 400°F and cook pasta according to the package directions. Stir occasionally, until pasta is al dente, about 10-12 minutes.
Cut the broccoli into bite-size pieces. Quarter the mushrooms and dice the onion. Add broccoli florets to the pasta, about a minute or two before the pasta is done.
When finished cooking, drain well in a colander and run under cold water to stop the cooking process.
Next, heat olive oil in a pan over medium-high heat. Add onion and sauté for 3 minutes, stirring occasionally. Add mushrooms and garlic, cook, until mushrooms are slightly golden brown.
Meanwhile, whisk cornstarch and 1/4 cup milk to a bowl and set aside.
Reduce the heat to a low and pour the stock and remaining 1 1/4 cup of milk into the pan. Add mustard, salt, and pepper, mix well to combine. Cook until the mixture starts to simmer.
Pour in the prepared cornstarch and milk mixture, mixing thoroughly until sauce is thickened.
Take off the heat and stir in 1 cup of the shredded cheese. Taste and adjust the salt and pepper according to your taste.
In a baking dish, add the cooked pasta, broccoli, and diced chicken. Pour over the mushroom sauce, and stir to nicely coat all the ingredients.
Smooth the top with the back of a spoon and sprinkle with the remaining 1/2 cup of shredded cheese. Bake in preheated oven for 15 minutes, until cheese on top, is bubbly and golden.
Serve warm and enjoy!