Set the oven to 350 degrees F. 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).
Take out a muffin tin 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.
Separate the meatloaf mixture into four equal portions in the bowl - this will help you keep the muffins of equal size. Take one portion, then separate it into four smaller portions and press each one into a muffin tin cup. Repeat with the other three larger portions. (Make sure to wash your hands with soap and warm water after touching the raw meat!)
If using a silicone muffin tin, place it on a cookie sheet then place it in the oven. If using a regular muffin tin, place it in the oven on its own. Bake for 10 minutes, then take the muffin tin out of the oven and measure out and place 1/2 tablespoon of ketchup on to each muffin and brush with a pastry brush or other utensil.
Put the muffin tin back in the oven and cook for another 25 minutes, turning the muffin tin 180 degrees halfway through the cooking time (particularly if your oven heats unevenly). Remove from the oven and let cool for 5 minutes. Either pop the meatloaf muffins out of the silicone tin or remove them with a spoon or other utensil from a metal tin. Serve and enjoy!