Preheat your oven to 400°F. Line a rimmed baking sheet with high-heat-resistant parchment paper.
In a medium bowl, mix the ground beef, mayonnaise, mustard, garlic, salt, pepper, parmesan, and parsley. Mix thoroughly but not excessively, and try to use a light touch. The more you work the meat, the tougher the meatballs.
Divide the mixture into 20 portions, each weighing about 1 ounce. I use a kitchen scale to make them uniform. But it's fine to eyeball it - divide the mixture into 4 parts, then divide each of these parts into 5 portions.
Shape the portions into 20 meatballs. Arrange them on the prepared baking sheet. Lightly spray them with olive oil.
Bake the meatballs until cooked through, for about 15 minutes.
If using the glaze, mix the ketchup and maple syrup in a medium saucepan while the meatballs are in the oven. 5 minutes before baking time is done, heat them over very low heat.
When the meatballs are done, turn the heat off, add them to the glaze, and turn to coat.
Transfer the meatballs to a serving plate or bowl and brush their tops with the remaining glaze. Serve immediately.
Video
Notes
You can replace the fresh garlic with 1 teaspoon of garlic powder and the fresh parsley with 1 teaspoon of any dried herb such as parsley, oregano, or thyme.
The nutrition info does not include the glaze. However, the glaze doesn't add significant calories or carbs. A serving with the glaze has 307 calories and 2 grams of carbs.
Extra-lean ground beef doesn't work well in this recipe. It produces dry meatballs. I recommend sticking with lean ground beef (85/15) or with fattier ground beef (80/20).
The meatballs don't emerge from the oven particularly browned. If you want them browned, use a broiler-safe pan and foil instead of parchment paper. Bake them for just 10 minutes, and finish them 6 inches under the broiler for around 1 minute (watch them closely!)