Mix breadcrumbs with milk in a bowl. Let sit for 2 minutes until fully soaked.
Combine Ingredients:
Put ground beef in a large bowl. Add soaked breadcrumb mixture, egg, Parmesan, garlic, parsley, salt, pepper, oregano, and red pepper flakes.
Mix Gently:
Combine with your hands for 1-2 minutes only. Don't overmix. Mixture should look loose, not compact.
Shape:
Using an ice cream scoop or hands, form golf ball-sized meatballs. Place on parchment-lined plate.
Chill:
Refrigerate for at least 30 minutes.
Brown:
Heat olive oil in a large skillet over medium-high heat. Add meatballs in batches (don't overcrowd). Cook 4-5 minutes, turning occasionally until golden brown.
Finish:
Pour marinara sauce into the pan with browned meatballs. Simmer 10-12 minutes until cooked through (internal temp 160°F). OR place on baking sheet and bake at 400°F for 8 minutes.