Bring a large pot of salted water to a rolling boil. Add pasta and cook until al dente according to package directions, usually 8-10 minutes. Drain and reserve 1/2 cup starchy pasta water.

Heat 2 tablespoons olive oil in a 12-inch skillet over medium-high heat. Season chicken with salt and pepper. Cook for 6-7 minutes, turning occasionally, until golden brown and internal temperature reaches 165°F when checked with instant-read thermometer. Remove and set aside.

In the same skillet, heat remaining 2 tablespoons olive oil over medium heat. Add chopped onions and sauté for 5 minutes until translucent, stirring occasionally.

Stir in minced garlic and cook for 1 minute, stirring constantly to prevent burning.

Pour in tomatoes, breaking them up with a wooden spoon if using whole canned tomatoes. Season with salt and pepper. Simmer for 15-20 minutes until sauce reduces and coats the back of a spoon.

Add cooked pasta and chicken to the skillet, tossing to coat. Add reserved pasta water if needed. Stir in diced mozzarella and 1/4 cup torn basil leaves, letting cheese soften for 1-2 minutes.

For baked version: Preheat oven to 350°F. Transfer mixture to a 9x13-inch baking dish, sprinkle with Parmesan cheese, and bake for 15-20 minutes until bubbly and golden.

Remove from oven and let rest for 5 minutes to allow sauce to settle. Serve garnished with additional fresh basil and a drizzle of olive oil if desired.