In a small skillet over medium heat, add 1/2 tablespoon of oil, diced onions, and garlic. Cook for 2-3 minutes until onions are translucent. Set aside to cool for 5 minutes.

In a large bowl, combine ground beef, bread crumbs, egg, Dijon mustard, Worcestershire sauce, salt, pepper, thyme, parsley, and cooked onions. Mix gently until just combined; avoid overmixing to keep the meatballs tender.

Shape the meat mixture into balls using a 1.5 tablespoon scoop and place them on a tray. Use your hands to shape and smooth the meatballs. Refrigerate the formed meatballs for 15-20 minutes to help them hold their shape during cooking.

In a deep 10 to 12-inch skillet, heat 4 tablespoons of vegetable oil over medium heat. Add the meatballs in batches and cook for 5-7 minutes, rotating to brown on all sides. Cooking in batches prevents overcrowding, ensuring even cooking.

Once all meatballs are cooked, remove them from the skillet, place on a tray, and cover with foil to keep warm.

In the same skillet, melt 2 tablespoons of butter, then add sliced onions. Cook over medium heat until onions soften and start to brown, about 8-10 minutes. Stir occasionally to avoid burning.

Add minced garlic and cook for an additional 2 minutes, stirring until fragrant.

Add flour to the skillet and mix to form a roux. This will help thicken your gravy nicely. Add salt, pepper, beef broth, and Worcestershire sauce. Bring to a boil and cook for about 6 minutes to thicken.

Reduce heat to low, add the meatballs to the gravy, and cook for 15-20 minutes to heat the meatballs thoroughly and allow them to absorb the delicious flavors of the gravy.

Serve warm over whipped mashed potatoes with the pan gravy. Optionally, garnish with fresh chopped parsley for a burst of color and flavor.