Pat chicken breasts dry with paper towels and season both sides evenly with kosher salt and black pepper.
Set up three shallow dishes: one with all-purpose flour, one with lightly beaten eggs, and one with Italian seasoned breadcrumbs mixed with grated Parmesan cheese.
Dredge each chicken breast in flour, shaking off excess, then dip in egg, and finally coat thoroughly in breadcrumb mixture, pressing gently to adhere.
Heat olive oil in a large skillet over medium-high heat and brown chicken for 2-3 minutes per side until golden but not cooked through.
Spread 1 cup marinara sauce in the bottom of a 6-quart slow cooker, arrange browned chicken breasts in a single layer, then top with remaining sauce and shredded mozzarella cheese.
Cover and cook on LOW for 4-5 hours or HIGH for 2-3 hours until chicken reaches 165°F internal temperature and cheese is melted and bubbly.
Let rest for 10 minutes before serving garnished with fresh basil.