If you're serving the meatballs with mashed potatoes, fill a medium pot with water about 3/4 full and cover with a tight-fitting lid. Heat this over high heat to bring to a boil. You want just enough water to cover the potatoes during cooking. While the water is heating, peel and dice the potatoes. Remember: Smaller potatoes cook faster thank large chunks. Add these to the pot even if the water is not boiling yet. Be sure to salt the water liberally.
For the Meatballs:
Into a large bowl, add the bread crumbs, milk (or water), egg, onions, ketchup, mustard powder, Worcestershire, salt, and ground black pepper. Mix well then add the ground beef and with clean hands mix the ground beef with the breadcrumb mixture.
Add two tablespoons of olive oil into a large non-stick (Teflon coated) skillet set over medium-high heat.
Shape meat mixture into 1-1/2 inch meatballs. Add the meatballs to a large Teflon coated skillet. If you have a smaller skillet, you may need to cook in batches to avoid overcrowding the pan. Overcrowding leads to steaming rather than browning the meat. Brown the meatballs on all sides.
Once browned, move the meatballs around the edge of the pan and start the base of the gravy in the middle area of the skillet. If you have a smaller skillet, you may need to remove the meatballs to a plate and set aside.
Add butter to the center of the skillet and melt completely. Add onions and mushrooms and cook until onion is soft and translucent - about 3 minutes.
Sprinkle the cornstarch over the onions and mushrooms, then stir to coat. I find this helps with clumping. Add the beef broth and stir to combine working out any lumps you find.
Move the meatballs around back into the full pan. Season the gravy with salt and pepper and bring to a full boil. Cornstarch needs to boil to activate thickening. After about 3 to 5 minutes, the sauce should begin to thicken.If the sauce becomes too thick add a bit more beef broth or water. If the sauce is too thin, mix another 1 tbsp of cornstarch into 1 tbsp of cold water and stir to combine. Then, while whisking, slowly drizzle this mixture directly to the sauce to thicken. Taste and adjust seasonings, if necessary.
If you're serving with potatoes, they should be just about tender and ready to mash.
Serve meatballs garnished with fresh chopped parsley or thyme.
Salisbury Steak Meatballs with Mushroom Gravy Video
You may substitute 1/4 cup prepared whole grain, stone ground or dijon mustard for the tablespoon of dry mustard powder.
Worcestershire sauce makes beef taste beefier. It really amplifies the flavor. You can omit the Worcestershire sauce if you prefer.
You may substitute cornstarch for the flour if you prefer. Cornstarch is Gluten-free.