Pour a small amount of the jarred marinara into the bottom of a slow cooker and spread it around to act as a bed. Add in the chicken tenders, onions, garlic, the rest of the marinara, Italian Seasoning, bay leaves, crushed red pepper flakes, salt, pepper, and finish with the cream cheese.
Put the lid on the slow cooker and cook on low for 6 hours or on high for 3.
When the time is up, cook the pasta according to the package directions.
Remove the chicken from the slow cooker and shred, then add back in.
Stir in the spinach, basil, and cooked, drained pasta. Taste for seasoning. Serve topped with shredded mozzarella.