In a medium bowl, combine the chicken (or turkey), eggs, onion, breadcrumbs, Italian seasoning, garlic powder, ½ cup marinara and ½ cup cheese (it's easiest to just use your hands to do this).
Spray an 8x5 inch loaf pan with nonstick spray (metal or glass- see notes below). Scoop the meatloaf mixture into the pan and smooth the top. Spread the remaining ½ cup of marinara sauce over the top. Sprinkle remaining 1 cup of cheese on top.
Put the pan in a 6 quart slow cooker and place the lid on top. Cook for 7 to 8 hours on low (or 4 hours on high). Slice, and serve.
Notes
Note: I used a glass pan and cooked mine on LOW. If you're opting for cooking it on HIGH, you might consider using a metal pan instead. I'm not certain the glass would work well with such a high temperature.
If you are preparing this recipe as GLUTEN-FREE, be sure to use a sub for the regular breadcrumbs (like GF breadcrumbs), and be sure to use a brand of marinara sauce that is known to be GF (Classico Tomato Basil is GF).
Option: Place some small potatoes around the sides of the pan so that they can be slow cooked along with the meatloaf.