In a deep 12″ skillet or wide dutch oven, heat oil over medium-high heat. Add chicken, mushroom, and onion. Stir fry until chicken is mostly cooked through, about 5 minutes. Stir in garlic, and cook one minute longer.
Add pasta to the pot, and pour in chicken broth. Stir mustard into the water, and pour that into the pot.
Stir everything together, trying to make sure all pasta is covered with liquid. Bring to a boil. Continue boiling, stirring frequently, for the recommended cooking time on the pasta directions.
Reduce heat to low. Stir in ham and cheese, and continue cooking, stirring almost constantly, until cheese is melted (approximately 3 minutes). Serve immediately.
If you would like breadcrumbs on top of this pasta dish, stir together 1/2 cup breadcrumbs and 2 tablespoons melted butter. Sprinkle buttery breadcrumbs over the top of the pasta in the skillet. Place pan under hot broiler for 3-5 minutes to brown breadcrumbs. Keep a close eye on the dish while broiling to avoid burning the top. (This step requires an oven-proof skillet!)
When reheating leftovers, add 1-2 tablespoons of water or milk per serving to loosen up the sauce.