Place breadcrumbs in a bowl. Use a box grater and grate the onion over the breadcrumbs. Mix with fingers, leave to soak for a few minutes.
Add remaining steak ingredients into the large bowl. Use your hands to mix until just combined.
Divide into 5 and pat into oval patties around 3/4" / 1 2/3 cm thick.
Heat oil in a skillet over high heat. Add the steaks and cook the first side for 1 minute or until browned, then turn and brown the other side (they will still be raw inside). Remove onto plate.
Add chopped onion and garlic and cook for 2 minute until onions are a bit translucent.
Add the mushrooms into the skillet and cook for 2 - 3 minutes until golden.
Turn heat down to medium. Add butter. Once melted, add flour and cook for 30 seconds, stirring constantly.
Gradually add in beef broth, stirring as you go. Whisk in remaining Gravy ingredients.
Add steaks along with the juices on the plate. Cook for 5 - 7 minutes, or until gravy is thickened, stirring occasionally around the steaks. If the gravy thickens too quickly, add more water.
Remove steaks onto a plate. Taste gravy and adjust salt and pepper to taste. Serve the steaks topped with the mushroom gravy with mashed potatoes or buttered egg noodles.