Add the bread crumbs and milk to a small bowl and soak for 20 minutes. It will form a paste, which helps with the meatball moisture as well as binding.
Heat 1 tablespoon olive a large pan to medium heat and add onion and optional peppers with a bit of olive oil. Cook for 10 minutes, or until very soft and starting to caramelize. Add garlic and stir. Cook for 1 minute until it smells good! Remove from heat and cool.
In a large mixing bowl, add beef, veal (and/or pork), bread crumb mixture, cooked onion mixture, Parmesan cheese, Italian herbs, red pepper flakes, salt and pepper, and optional SPICY ingredients. Hand mix to combine, but not too much. Overmixing will make it become mealy.
Form into 16 2-ounce meatballs and refrigerate, covered, until ready to use.
Bring your large pan back to medium-high heat and add 2 tablespoons olive oil. Add meatballs and sear them about 1 minute per side, rolling them around a bit to get most of the sides. Get them nicely browned. Drain some of the grease if desired.
Add the meatballs to your simmering spicy marinara sauce and simmer 20 minutes, or until cooked through.