This meatloaf cooks directly in the slow cooker, so for easy removal you want to line the slow cooker with foil strips that you can use to lift it out when it's done.
I used my 6-Quart oval slow cooker but any size from 3-1/2 to 6-Quart should work.
Cut a 2-foot length of foil in half lengthwise. Fold each strip in half lengthwise to form two long strips. Place the foil strips in the bottom of the slow cooker in a cross pattern. Trim them to be even with the rim, so they don't get in the way of the crock pot cover.
Spray the crockpot and foil with nonstick spray.
In a large bowl, combine the onion, celery, carrot, egg, beef, oats, ketchup, garlic powder, thyme, salt and pepper. Use your hands to to blend everything together well. Place the mixture in the foil-lined crockpot and form it into an even shaped loaf as best you can.
Cover and cook on LOW for 5 to 7 hours, or until a meat thermometer inserted into the center of the meatloaf reads 170F.
Turn off the slow cooker and carefully grasp the ends of the foil strips to lift out the meatloaf. Let the meatloaf rest for about 15 to 20 minutes to let it firm up before trying to slice it.