In a small skillet, sauté the onion, garlic and celery until soft. Set aside to cool.
In a large bowl, combine the meat, egg and breadcrumbs.
Pour the milk into a separate bowl and submerge each piece of bread. Lightly squeeze out the excess milk, then tear each slice into pieces and add to the meat.
Add in the marinara, parmesan and seasonings.
Add in the sautéed onion, garlic and celery from earlier too.
Mix everything together (your hands are the best tool for this), then try to make a tester like I mentioned before.
If you're satisfied with the seasoning levels, start to roll the meat mixture into 2 to 3 inch sized balls.
Get two pans going on the stove over medium to low heat and start to fry up the meatballs.
You'll want to rotate the meatballs in the pan to get an even cook on them and make sure they aren't sticking.
You should see a nice brown color on all sides of the meatball (this part can be time consuming but it's what makes this recipe so good, it should take about 20-25 minutes total).
When the meatballs are fully cooked, transfer them to a paper towel lined baking tray to get rid of any excess oil.
My favorite ways to serve are in a sandwich, alongside pasta, or on top of creamy polenta!