Combine milk, pasta, salt, and butter in a dutch oven or heavy saucepan. Bring to a low boil over medium heat. Note: stir constantly to prevent the milk from scalding on the bottom of the pan.
After 3 minutes of cooking, add the onion, and continue to cook the pasta for another 3 minutes.
1/3 cup white onion
Reduce heat to low and add the garlic powder, cubed cream cheese, black pepper, and Parmesan. Stir over low heat for 1 minute and then turn the burner off and place a lid on the dutch oven and let it sit for 2 minutes. Stir until the cream cheese is fully incorporated.
2 tsp garlic powder, 8 oz cream cheese, 1/2 tsp ground black pepper, 1 1/3 cup shredded Parmesan
Add the chicken and broccoli to the pot and stir to incorporate into the pasta.
1 lb chicken breast, uncooked, 3 cups broccoli
Top evenly with the breadcrumb mixture.
Bake for 30 minutes, checking at 20 minutes to make sure the breadcrumbs are browning evenly.
If you prefer a crispier topping, you can place the pot under a broiler set to low for 1-2 minutes, watching carefully to avoid burning.
Notes
If you want to use less milk, you can parboil the noodles and onion in salted water with olive oil, cooking for 6 minutes. Drain.Combine 2 cups milk, cream cheese, and salt in a medium saucepan and whisk on medium-low heat until the cream cheese has melted. Add the Parmesan and black pepper and stir until combined. Add the noodles to the pot, and start with step 6 above.