Step 1: In a large bowl, combine ground beef, breadcrumbs, egg, finely chopped onion, the first tablespoon of Worcestershire sauce, garlic powder, salt, and black pepper. Gently mix until just combined, being careful not to overmix. Form the mixture into 6-8 oval-shaped patties, about 3/4-inch thick.
Step 2: Heat the tablespoon of olive oil in a large skillet or Dutch oven over medium-high heat. Add patties and cook for about 4-6 minutes per side, until beautifully browned. Remove patties and set aside, leaving drippings in the pan.
Step 3: In the same skillet, melt the tablespoon of unsalted butter over medium heat. Add thinly sliced onion and cook, stirring occasionally, until soft and golden-brown (about 5-7 minutes).
Step 4: Add sliced mushrooms and cook for another 5-7 minutes until tender. Stir in minced garlic and cook for just 1 minute more until fragrant.
Step 5: Sprinkle flour evenly over the onions and mushrooms. Stir constantly for 1-2 minutes to cook the flour.
Step 6: Gradually whisk in the beef broth, a little at a time, stirring continuously to prevent lumps. Stir in the second tablespoon of Worcestershire sauce and the Dijon mustard. Bring the gravy to a gentle simmer, stirring until slightly thickened.
Step 7: Carefully return the browned patties to the skillet, nestling them into the gravy. Reduce heat to low, cover, and simmer gently for 15-20 minutes, allowing the patties to cook through and the gravy to thicken.
Step 8: Taste the gravy and adjust seasoning with salt and black pepper as needed.
Step 9: Serve hot, smothered in gravy. Garnish with fresh chopped parsley if desired.