Cook pasta in lightly salted water according to the instructions on the package. When ready drain but keep a little of the cooking water.
Heat cooking oil over high heat and cook chicken pieces until golden. Transfer to a bowl or plate.
In the same pan, saute onion over medium heat. Stir regularly. After 2-3 minutes, add garlic and cook for 2-3 min more.
Add cherry tomatoes. Cook for 5-8 minutes or until softened.
Add balsamic vinegar and cook for 1 minute more.
Combine cooked/drained pasta, tomatoes, spinach, salt, and pepper.
Cook on medium heat until spinach is wilted. If too dry add a little of the pasta cooking water.
When ready stir in mozzarella and basil.
Serve.