Add all the meatball ingredients (excluding the olive oil) to a large bowl and mix well, use your hands. Shape into 1 inch meatballs. You should get about 30 meatballs.: Close your eyes for a moment and notice the cool tacky texture as your hands fold the mixture together, the smell of beef blended with the tang of mustard and Worcestershire sauce . The goal here is gentle cohesion, not a dense paste, so press just enough to shape each little sphere; overmixing will make them tight and chewy. When forming 1 inch meatballs, watch for smooth surfaces and even sizes so they cook uniformly, and keep a tray nearby for resting them before the pan. A common pitfall is compacting them too hard, which yields dry meatballs, so handle lightly and refrigerate briefly if the mix becomes too soft to shape.
Add the olive oil to a large skillet and heat over medium to high heat. Add meatballs and cook. If you're using a large skillet all meatballs should fit in one batch otherwise, you might have to fry them in a couple batches. Cook the meatballs on all sides, add more oil if needed. Remove meatballs from skillet and set aside.: You will hear a pleasant sizzle when the first batch hits the hot oil, and that sound signals the Maillard reaction that builds crust and flavor. The surface should turn deep golden brown in spots, offering a contrast between the crisp exterior and tender interior. Work in batches if needed to avoid overcrowding, which steams instead of sears. Use tongs to rotate each meatball so all sides brown, and transfer them to a paper towel lined plate to rest. A frequent mistake is leaving the heat too low, resulting in pale, floppy balls, so keep the pan hot but not smoking to achieve even browning.
To the same skillet add butter and melt. Add onion and cook until onion is soft and translucent.: As the butter melts it will lift the fond from the pan, then adding the chopped onion releases sweet, fragrant steam as it softens. Stir often and watch the color shift from raw white to a warm, translucent glow with the faintest edges of gold. This slow gentle cook brings out sweet complexity that will underpin the gravy, so resist upping the heat to rush the process or you risk burning. If the onions start to brown too quickly, lower the temperature and continue stirring, because bitter bits will carry through to the sauce.
Sprinkle the flour over the onion and stir. Cook for about 1 minute to remove the raw flour taste. (If using cornstarch make a slurry with equal amounts water and cornstarch) Add broth and remaining gravy ingredients to skillet, and cook for about 3 to 5 minutes, sauce should thicken. If the sauce is too thick add a bit more beef broth. Taste for seasoning and adjust as necessary.: When you sprinkle the flour into the warm onion, it should coat the pieces and start to smell toasty as you stir, a sign the raw taste is evaporating. Slowly pour in the beef broth while whisking or stirring to prevent lumps, and watch the sauce transform from thin to a glossy, coat the back of a spoon consistency. You will notice the aroma deepen and the viscosity increase; this is the gravy coming together. Adjust texture with more broth if it gels too much, and always taste for salt and balance since reduced broth concentrates flavors. One misstep is adding cold broth too quickly, which can create lumps, so add it gradually while stirring.
When the gravy is done add the meatballs back to the skillet and toss them around so they're covered in the gravy. Garnish with parsley.: When you return the browned meatballs to the pan, you should hear a soft simmer as they nestle into the sauce, and the gravy will cling to their seared surfaces, marrying flavors. Use a gentle spoon to turn each meatball so every side gets coated, then let them warm through for a few minutes so flavors permeate. The final step is the bright sprinkle of chopped parsley , which lifts the rich sauce with a fresh note. Avoid boiling aggressively at this stage, which can toughen the meat, instead maintain a low simmer for a tender finish.
