First, oil the basin of the slow cooker with about 2 Tbsp of Olive Oil. This will both help form a crust, and prevent the chicken from sticking to the sides of the dish.
Rinse and pat dry your chicken breasts - you should have about 5 -6 in total. Season liberally with salt and pepper. In a shallow bowl or pie plate, combine the egg with 1 Tbsp of water, whisking to combine with a fork. In a second shallow bowl, mix the breadcrumbs and grated parmesan cheese. Working with one chicken breast at a time, dip in the egg wash on both sides, and then into the crumb mixture, pressing to adhere on all surfaces. Place the chicken breasts like a puzzle into the bowl of the slow cooker. It's fine if they overlap, but they should cover the base fully.
When the chicken is all in the basin, top with a jar of Rao's Marinara, or sauce of your choice. No need to mix, as this dish cooks in layers and the sauce helps seal the chicken for tender cooking below. On top of the marinara, add a thick layer of mozzarella, top with oregano and red pepper flakes, and put the lid on! Cook on HIGH for 3-4 hours - I tend to find it done around 3 hours or so, but depending on your slow cooker, you may need four hours.
When ready to serve, dig into the chicken and pull out by the spoonful from the top - the chicken will be fall apart tender, so on the plate this won't look like a traditional chicken parmesan breast, but trust me it's DELICIOUS! Serve with extra grated parmesan and freshly torn basil.