Preheat the oven to 400°F.
If making homemade breadcrumbs, start there. Tear the sourdough into pieces and toss with olive oil and Italian seasoning. Spread on a baking sheet and bake for about 15 minutes, until crisp. Let cool slightly, then crush into coarse crumbs.
Add the ground beef, Italian sausage, minced onion, cheese, eggs, milk, breadcrumbs, parsley, red wine vinegar, salt, and pepper to a large bowl. Gently mix just until combined. Try not to overmix.
Roll the mixture into golf ball sized meatballs and place them on a parchment lined baking sheet.
Bake the meatballs for 18 to 22 minutes, until cooked through and lightly browned.
While the meatballs bake, warm the marinara sauce in a large skillet or saucepan over low heat.
Add the baked meatballs to the marinara and let them simmer for about 10 minutes.