Gently combine all ingredients (except ketchup) in a large bowl. Just be careful not to over-mix, or the meat will be tough. I like to use my hands for this!
Shape beef mixture into a loaf and place in the prepared slow cooker.
Spread a thin layer of ketchup over the top of the meatloaf.
Cover and cook on LOW for 4-6 hours or on HIGH for 2-3 hours, or until a meat thermometer inserted in the center of the meatloaf reads 160°F.
Remove meatloaf from slow cooker. Place on a cutting board, slice and serve.
TO BAKE IN THE OVEN:
Place the prepared meatloaf on a foil lined baking sheet that has been sprayed with cooking spray. Bake, uncovered, at 350°F for about 1 hour.
Notes
Use 85%-93% lean ground beef, which works well to keep the meatloaf moist and juicy. Leaner ground beef has a tendency to dry out in the oven or Crock Pot, while fattier beef can be greasy.
Use your hands or a fork to gently combine the meatloaf mixture. Be careful not to over-mix the meat, or it will become tough and dry.
Every brand/type of slow cooker runs at slightly different temperatures. If you have a Crock Pot that cooks pretty hot, your meatloaf will likely only require about 2 hours on HIGH or 4 hours on LOW. That's why a meat thermometer is such a great tool -- you can turn off the slow cooker when the meal is done.
You'll know the meatloaf is done when it registers aninternal temperature of 160°F and the juices run clear.