01 - Lightly grease the inside of a 5 to 6 quart slow cooker with cooking spray or a thin coating of oil.
02 - Spread the cubed chicken breasts evenly across the bottom of the slow cooker. Season with salt and freshly ground black pepper.
03 - Scatter the finely chopped onion and minced garlic over the seasoned chicken layer.
04 - Layer the halved cherry tomatoes and roughly chopped baby spinach on top of the aromatics.
05 - In a mixing bowl, whisk together the heavy cream, low-sodium chicken broth, and basil pesto sauce until smooth and well combined.
06 - Pour the pesto cream mixture evenly over the layered ingredients in the slow cooker.
07 - Sprinkle 1 cup of the shredded mozzarella and 1/4 cup of the grated Parmesan over the sauce.
08 - Stir gently to combine all ingredients. Cover and cook on the LOW setting for 3 hours.
09 - After 3 hours, gently fold in the refrigerated cheese tortellini until evenly distributed throughout the casserole.
10 - Re-cover and continue cooking on LOW for another 45 minutes to 1 hour, until the tortellini are tender and the chicken is cooked through to an internal temperature of 165°F.
11 - Scatter the remaining mozzarella and Parmesan over the top. Cover and let stand for 5 minutes until the cheese is fully melted.
12 - Finish with a sprinkle of crushed red pepper flakes and torn fresh basil leaves, if desired. Serve directly from the slow cooker.