Pat steaks dry with paper towel. Season with salt and pepper to your taste.
Heat a lightly oiled skillet or pan (or barbecue) on high heat until just beginning to smoke. Cook steaks for 3 minutes each side or until cooked to your liking. Rest for 5 minutes.
Heat remaining oil in the skillet. Add onion and cook for 2-3 minutes until onions are translucent, then add the garlic and cook for a further 30 seconds, or until fragrant.
Add the sliced mushrooms into the skillet and cook for 3 minutes until golden and beginning to soften.
Reduce heat to medium and melt butter in the skillet. Add flour and cook while stirring for 1 minute, allowing the flour to brown slightly.
Slowly and gradually add in the beef broth (or stock), while stirring. Allow to simmer for 4-5 minutes, or until thickened. Stir in the Worcestershire sauce and season with salt and pepper to taste.
Add the steaks back into the gravy in the pan along with the juices on the plate OR serve steaks with the onion mushroom gravy on the side. Garnish with chopped thyme, parsley or rosemary if desired.
Notes
If you want to get the most flavour out of your steaks, you can choose to add your steaks back into the pan and allow the gravy to seep into them OR serve them with gravy on the side. If you prefer your steaks to stay juicy and you won't be serving it straight away, leave them on a plate until ready to serve so they don't dry out.