Pound the chicken breasts to an even thickness and season them lightly with salt and pepper.
Set up three shallow dishes: one with flour, one with beaten eggs, and one with panko breadcrumbs mixed with Italian herbs if desired.
Dredge each chicken piece first in flour, then dip into the eggs, and finally coat thoroughly with panko breadcrumbs, pressing lightly to adhere.
Heat olive oil in a skillet over medium heat until shimmering and fragrant. Sear the breaded chicken for 2-3 minutes per side until golden brown, just to develop color and flavor.
Transfer the seared chicken to the slow cooker, arranging them in a single layer or slightly overlapping.
Pour the marinara sauce evenly over the chicken, covering about two-thirds of each piece.
Sprinkle shredded mozzarella and grated Parmesan evenly over the sauce, creating a cheesy layer that will melt and bubble during cooking.
Cover the slow cooker and cook on LOW for 4-5 hours until the chicken is tender, easily shredded, and the cheese is bubbly and golden.
Once cooked, turn off the slow cooker and let the dish rest for about 5 minutes to allow flavors to meld and cheese to set slightly.
Garnish with fresh basil leaves, then serve the chicken hot with pasta or crusty bread, spooning sauce and melted cheese over each plate.