Place a box grater into a large bowl. Using the largest holes, grate the onion into the bowl.
Add the egg to the bowl, and whisk until combined. Add the garlic, breadcrumbs, parmesan, salt, pepper and mix well.
Add the ground beef and gently mix until the breadcrumb-cheese mixture is well incorporated, being careful not to overmix.
Gently form 24 meatballs (about 1 ½ ounces each).
Pour half (1 ½ cups) marinara into a 6-quart slow cooker.
Gently layer the meatballs into the sauce so they are all in a single layer in the bottom of the slow cooker.
Pour the remaining (1 ½ cups) marinara over the top. Do not mix or the meatballs could fall apart.
Cover and cook on high for 3-4 hours, or low for 6-8 hours, or until the meatballs are cooked through and every tender.
Serve with fresh basil and freshly grated parmesan cheese, over cooked spaghetti or pasta if that's your thing.
Notes
For a more traditional Italian meatball, swap half the ground beef for ground pork, or mild Italian sausage works in place of plain ground pork, too.
For perfect spaghetti with these meatballs, bring 4 to 6 quarts of well-salted water to a rolling boil, add the 1 pound of spaghetti, and stir immediately to prevent clumping; cook to al dente per package directions (usually 9 to 11 minutes). Reserve 1/2 cup of pasta water before draining, then toss the spaghetti directly in the slow cooker marinara with that splash of starchy water to help the sauce coat every strand. YUM!
Use gluten free breadcrumbs for a gluten free diet.