Season chicken breasts evenly with salt and black pepper.
Place the seasoned chicken in a single layer at the bottom of the slow cooker.
In a small bowl, whisk together the marinara sauce, minced garlic, and Italian seasoning.
Pour the sauce mixture over the chicken, ensuring each piece is fully covered.
Cover and cook on high for 3 to 4 hours or on low for 6 to 7 hours until chicken reaches an internal temperature of 165°F (74°C).
Remove the lid and sprinkle the mozzarella and parmesan cheeses over the top of the chicken.
Replace the lid and cook for an additional 10 to 15 minutes, or until the cheese is completely melted and bubbly.
Garnish with fresh basil and serve immediately.