Preheat the oven to 475°F and position the rack in the upper-middle position.
In the bowl with the reserved onion mixture, mash the bread and milk until smooth.
Add the remaining ingredients (except beef) and mash to combine.
Add ground beef and knead until well mixed.
Shape into 2½-inch meatballs (about 16 total) and place on a rimmed baking sheet.
Bake until well browned, about 20 minutes.
Finish and Serve
Transfer baked meatballs to the marinara sauce. Simmer for 15 minutes.
Serve over pasta.