Place breadcrumbs in a large mixing bowl.
Grate the onion directly over the breadcrumbs to catch the juices and let soak for five minutes.
Combine ground beef, garlic, egg, ketchup, bouillon, Worcestershire sauce, and Dijon mustard into the bowl with the breadcrumb mixture.
Mix by hand for two minutes until the texture is consistent and pasty.
Divide into five equal portions and form into oval patties about 3/4 inch thick.
Heat olive oil in a skillet over medium-high heat and sear patties for 3-4 minutes per side until browned; remove from pan.
In the same pan, sauté mushrooms until softened.
Whisk flour into the pan drippings, then slowly add beef broth while whisking to create a smooth gravy.
Return patties to the skillet and simmer in the gravy for 5-8 minutes until cooked through.