Season the chicken breasts on both sides with salt, pepper, and garlic powder.
Place the chicken breasts in the slow cooker.
Pour the marinara sauce over the chicken, ensuring they are well covered.
Sprinkle Italian seasoning evenly over the top.
Cover the slow cooker and cook on low for 6 hours or until chicken is cooked through and tender.
About 30 minutes before serving, sprinkle the shredded mozzarella and grated Parmesan cheese over the top of the chicken.
Re-cover the slow cooker and continue cooking until the cheese is melted and bubbly.
Once ready, remove the chicken and serve hot, garnished with fresh basil if desired.
For added flavor, sauté onions and bell peppers and add them to the slow cooker with the chicken and sauce.
Serve the cheesy chicken over pasta, rice, or with crusty bread for a complete meal.