Grate onion over breadcrumbs in a large bowl. Let sit 5 minutes to absorb moisture.
Add remaining patty ingredients except olive oil to the bowl. Mix gently with your hands until just combined. Don’t overmix.
Shape mixture into 6 oval patties, about 1-inch thick. Press a slight dimple in center of each.
Heat olive oil in a large skillet over medium-high heat. Sear patties 3-4 minutes per side until nicely browned. Remove to a plate.
In same skillet, melt butter. Add diced onion and cook until softened, about 3 minutes.
Add mushrooms and garlic. Cook until mushrooms release their moisture and start to brown, 5-7 minutes.
Sprinkle flour over vegetables, stir and cook 1 minute.
Gradually whisk in beef broth, Worcestershire sauce, and Dijon. Bring to simmer.
Return patties to skillet, including any juices from plate. Cover and cook 10-12 minutes until patties are cooked through and gravy thickens.
Season gravy with salt and pepper to taste. Garnish with parsley if desired.
Notes
Don’t skip grating the onion – it’s key for tender, flavorful patties
Make ahead: Form patties up to 24 hours ahead, store covered in refrigerator
For best results, use room temperature beef and eggs
If gravy gets too thick, thin with additional beef broth
Store leftovers covered in refrigerator up to 3 days