Cook pasta according to package directions.

Heat olive oil over medium-high heat. Add chicken pieces in a single layer and sprinkle with salt and pepper. Allow to cook without stirring for 2 minutes, or until golden.

Stir chicken around to brown on all sides. Remove to a plate.

In the same skillet over medium-high heat, add the onions and garlic and stir immediately to start cooking.

Cook for 2 to 3 minutes, stirring occasionally, until the onions are deep golden.

Reduce the heat to low and pour in the marinara sauce, red pepper flakes, and water. Stir until onions and garlic are mixed in, then add chicken back into the pan.

Add parsley, stir to combine, then allow to simmer, stirring occasionally, for about 7-8 minutes.

Make sure the sauce is bubbly and very hot, then gently stir in the mozzarella.

Let it sit, undisturbed, for 2 to 3 minutes, until you can see that the mozzarella is very soft.