Chop the onion. Set aside 1/4 cup finely chopped for the steak mixture and reserve the rest for the gravy. Finely chop 1 cup of mushrooms for the patties and slice 2 and 1/2 cups of mushrooms for the gravy.

In a large bowl, combine the ground beef, 1/4 cup finely chopped onion, 1 cup finely chopped mushrooms, egg, panko breadcrumbs, tomato paste, mustard, garlic, Worcestershire sauce, liquid smoke if using, salt, pepper, and onion powder. Use your hands to mix until just combined. Stop as soon as everything comes together to avoid tough patties.

Divide the mixture into 6 equal portions and shape each into an oval patty about 3/4 inch thick.

Heat a large skillet over medium-high heat. Add 1 tbsp oil and let it shimmer. Sear the patties in batches for about 2 minutes per side until deep golden brown. Do not crowd the pan. The centers do not need to be fully cooked at this stage. Transfer to a plate and set aside.

In the same skillet, melt 1 tbsp butter over medium heat. Add the reserved sliced mushrooms and chopped onion. Saute for 5 minutes until softened. Add the garlic and cook for 1 more minute.

In a measuring cup, whisk together the beef broth and flour until completely smooth. Pour the mixture slowly into the skillet while whisking constantly. Add 1 tsp mustard and 1 tbsp Worcestershire sauce. Stir and cook over medium-high heat for 2 to 3 minutes until the gravy simmers and thickens enough to coat a spoon.

Nestle the patties back into the skillet. Reduce heat to a gentle simmer, cover loosely, and cook for 5 more minutes until the patties are fully cooked through with no pink remaining. Spoon gravy over the top, garnish with fresh parsley, and serve right away.