Shred your chicken. Two forks work well, but I prefer to simply rip it with my fingers.
Dice your onion and mince your garlic.
Preheat your oven to 400 degrees.
Melt the butter in a large (at least 3.5 quart) oven safe skillet and cook the onion and garlic slowly until translucent.
Cook your pasta in 6 cups of boiling water with the chicken base or bouillon added to it.
Whisk the flour together with 1 cup of the milk.
Add the remaining 3 cups of milk to your skillet and bring it to a low boil on the stove over medium-high heat, stirring frequently.
Reduce the heat to medium and whisk in the flour/milk mix until smooth. Cook until thickened.
Stir the cooked pasta and broccoli into the milk mixture and cook for a couple of minutes.
Add the seasoning and stir through.
Sprinkle the mozzarella over the top and allow it to melt.
Add the chicken, and stir through until all of the cheese is melted. Remove from the heat.
Sprinkle the Parmesan evenly over the top of the casserole, followed by the fried onions.
Bake in your per-heated oven for 15 minutes.
Turn your oven to broil, and broil for about 2 minutes until the top is golden brown.
Remove and let stand for 5 minutes before serving.