Heat 2 tbsp oil in Dutch oven over medium heat until shimmering.
Cook onion and ¾ of the garlic until softened, about 4 minutes.
Stir in penne, crushed tomatoes, water, 1½ tsp salt, and red pepper flakes.
Bring to a boil, reduce heat, cover and simmer until pasta is tender, about 10 minutes.
In a small bowl, combine panko, remaining oil, remaining garlic, and a pinch of salt.
Microwave, stirring occasionally, until golden brown, about 2-3 minutes.
Preheat the broiler and adjust the oven rack to the middle.
Stir chicken into pasta, sprinkle with mozzarella.
Broil until cheese is melted and golden brown, about 5 minutes.
Top with toasted panko mixture and serve immediately.