Preheat the oven to 375°F (190°C).

In a large bowl, sprinkle the diced chicken with Italian seasoning, garlic salt, and pepper. Mix well.

Heat the olive oil in a skillet over medium-high heat. Add the seasoned chicken and cook for 3 minutes.

Add the broccoli florets to the skillet and cook for another 3-4 minutes, until the chicken is golden and cooked through, and the broccoli is tender.

Bring a pot of salted water to a rolling boil and cook the rotini one minute less than al dente, as it will continue cooking in the oven.

In a large mixing pot, combine the cream of mushroom soup, sour cream, milk, and sharp white cheddar until well mixed.

Drain the rotini, then gently fold it into the pot with the chicken and broccoli, blending evenly.

Grease a 9×13-inch casserole dish, then pour the mixture into the dish and spread it evenly.

Sprinkle the regular cheddar on top, cover with aluminum foil, and bake for 25 minutes.

Remove the foil, top with crispy fried onions, and bake uncovered for an additional 8 minutes, until golden and bubbly.