Prepare a slow cooker by lining it with heavy-duty aluminum foil. I recommend using two long pieces of foil and draping it over the slow cooker. The extra foil on the ends will help remove the meatloaf when finished cooking.
In a large bowl, combine the crushed saltines, ketchup, eggs, milk, salt, and pepper; stir until well-combined.
30 saltine crackers, 1/2 cup ketchup, 2 eggs, 1/2 cup milk, 1/2 teaspoon salt, 1/2 teaspoon pepper
Add the ground beef and diced onion; mix to combine.
2 pounds ground beef, 1/2 cup finely diced onion
Transfer the meatloaf mixture to the crockpot and shape into an oval with a slight dome. This will allow the fat drain off the sides instead of just pooling on top.
Combine the topping ingredients. Spread about 1/3 cup of the mixture over the meatloaf.
1 cup ketchup, 1/2 cup brown sugar
Cover with the lid then cook on low for 4-6 hours or on high for 2-3 hours or until the internal temperature is 160℉. Please note that every slow cooker is different and the time will vary!
Remove the meatloaf from the slow cooker by grabbing both sides of the foil carefully. Place the meatloaf on a rimmed baking sheet.
Preheat the oven's broiler.
Spread more of the ketchup mixture over the top and sides of the meatloaf.
Broil for 2-3 minutes, just long enough for the glaze to get warm and bubbly! Keep a close eye on it so it doesn't burn.
Let the meatloaf rest for 10 minutes.
Cut into slices and enjoy. Serve with any remaining ketchup mixture, if desired.