Pat the chicken breasts dry and season generously with salt and pepper on both sides.
In a small bowl, mix olive oil with minced garlic and red pepper flakes if you like some heat.
Place the chicken breasts in the slow cooker, then drizzle and rub the garlic oil mixture over each piece to coat evenly.
Pour the marinara sauce over the chicken, covering all pieces completely and avoiding overcrowding.
Set the slow cooker to low and cook for 4 to 5 hours, until the chicken is very tender and easily shredded.
Check the chicken after 4 hours; it should feel very soft when pierced with a fork and be bubbling gently in the sauce.
Carefully remove the chicken breasts using tongs and shred slightly with two forks if desired.
Transfer the shredded chicken to a baking dish, spreading it out evenly.
Sprinkle a generous layer of shredded mozzarella and grated Parmesan over the top of the chicken.
Broil in the oven at 200°C (400°F) for 3 to 4 minutes, until the cheese is bubbly and golden brown.
Remove from the oven, let rest for 5 minutes, then garnish with torn basil leaves and a drizzle of olive oil.
Serve hot with pasta, bread, or a fresh green salad, and enjoy the cheesy, saucy goodness.
