In a large bowl mix ground beef, egg, Worchestershire sauce, seasoned salt, dry mustard, pepper, dried onion, bread crumbs and tomato paste. I suggest mixing with your hands but you can use a wooden spoon if you prefer. Shape the meat mixture into 18-20 small meatballs.
Heat 2 tbsp olive oil in a large skillet and add the meatballs. Brown the meatballs on all sides and cook until done. Remove from the pan to a paper towel lined plate and set aside.
Drain excess grease from the pan leaving the brown drippings from the meatballs. This adds a punch of flavor to your gravy. Melt 2 tbsp butter in skillet. Add onions and saute stirring occasionally until they are golden brown (watch so they don't burn). Add 2 cloves minced garlic and saute for one more minute.
Whisk in 2 tbsp flour and cook until mixture starts to thicken about 1 minute. Stir constantly.
Add beef broth, Worchestershire sauce, ketchup, seasoned salt, mustard powder, and pepper. Stir and simmer until gravy thickens.
In the meantime, melt 1 tbsp butter in separate small skillet and add mushrooms. Saute until golden brown.
Add meatballs and mushrooms into the skillet with the gravy and stir to cover. Let simmer for 3 minutes to reheat meatballs. Garnish with chopped parsley if desired. Serve over buttered egg noodles or mashed potatoes.
Notes: *Meatballs can be made ahead and browned, then stored in the refrigerator or freezer. When ready to eat, make the gravy and then add meatballs and heat them up. *Leftovers can be stored in the refrigerator in a covered container for up to 4 days. They taste great and maybe even better the next day. *Meatballs and gravy can be stored in the freezer for up to 3 months. *To reheat warm on the stovetop and add a little beef broth if needed to thin the gravy out. You can also microwave them covered with a paper towel in short bursts of 20 seconds. *Serve over buttered egg noodles or my creamy mashed potatoes.