Heat your smoker or wood pellet grill to 225 degrees F while you prep. Peel then roughly chop the onion using a cutting board and large sharp knife. Place the onion into a food processor and process until diced. If you don’t have a food processor you can dice the onion by hand with the knife. Set aside (without removing the onion).
Take the frozen spinach out of the packaging (cutting away any excess if necessary). Let thaw in the fridge or use the microwave. Once thawed, place it in several paper towels and squeeze the excess water out over the sink. You may need to do this several times to get the water out.
Remove the spinach from the paper towel (some of it may stick to the towel, just pick those off with your fingers) and place it into the food processor in clumps, with the onion. Mix the two together in the food processor until well combined. Alternatively you can do this step by hand with a large sharp knife. Remove the spinach and onion mixture and place on several paper towels. Squeeze any remaining water out of them with your hands (over a sink).
Measure out a large piece of foil, roughly the size of a sheet pan, and set it aside. Place the ground beef into a large bowl and dump the spinach and onion mixture out of the paper towel and into the bowl. Measure out then add in the remaining ingredients to the bowl (minus the ketchup - leave that out). Using a fork or small whisk, combine the egg white and yolk together gently in the bowl, then using your hands, mix everything together well.
Take the meatloaf mixture out of the bowl and place it on the piece of foil. Use your hands to form it into a loaf that's approximately 10 to 12 inches long, about 4 inches wide, and roughly 2 inches tall. Slide the meatloaf onto a sheet pan or cutting board and carry it out to your grill or smoker. Slide the meatloaf with the foil onto the grill (without the sheet pan or cutting board) and, if you have it, put a temperature probe into the middle of the meatloaf. Set the probe temperature to 160 degrees F. Close the lid and let it smoke. The total cooking time will depend on the outside temperature, but it's around 3 hours.
At about the 2 hour mark (or roughly 145 degrees F internal temp), measure out the ketchup with a measuring spoon and put into a small bowl. Open the lid of the smoker or grill and, using a pastry brush, spread the ketchup all over the meatloaf. Close the lid and let the meatloaf finish cooking to 160 degrees F. Once the meatloaf is done, remove it from the grill and let cool for 10 minutes. Cut into pieces using a large sharp knife, then serve and enjoy!