Add the ground beef and bread crumbs to a large mixing bowl and mix to combine.
Add the remaining meatball ingredients to the mixing bowl and combine well, using your hands.
Form small balls with a tablespoon of the meat combo to form your meatballs.
Heat a large and deep pan over medium high heat then add in meatballs, leaving plenty of space around them (you may have to do this in batches). Brown all sides of your meatballs, about 4-6 minutes total time. Repeat until meatballs are all browned. Remove your meatballs and set them aside.
Add the butter to the same pan you cooked the meatballs in, then add in the onions and mushrooms once its melted. Saute for about 2-3 minutes.
Sprinkle in the flour and gently stir to coat the mushrooms and onions, then continue cooking for another minute. Lower the heat if necessary.
Next, slowly pour in the beef broth and gently stir to ensure the flour has dissolved, is well blended and begins to thicken slightly. Season with Worcestershire, black pepper and sea salt.
Return meatballs to the pan with the savory mushroom sauce and allow the meatballs to cook in the Salisbury sauce for about 10-15 minutes, or until beef is no longer pink in the center.
Notes
If you're doubling the sauce or want it thicker, remove ¼ cup of current sauce and mix it with about 2 tablespoons of arrowroot powder or non-GMO corn starch in a bowl, whisking it well to ensure there are zero lumps, then add it back into the pan for a thicker sauce, then allow the Salisbury meatballs to cook at least another minute or two.
For smooth round meatballs, have a bowl of water near you to lightly dampen your hands before rolling meatballs, this helps create a very smooth look to the meatballs and keeps them from sticking to your hands.