Heat the oil in a large frying pan over a medium/hot flame. Cook the onions until they soften.
Add the crushed garlic, stir around the pan and then immediately add the steaks. Push down onto the onion & garlic for a few seconds, then turn over and do the same to the other side.
Cook the steaks for 3 (rare) to 5 (well-done) minutes on each side. While they're cooking, combine all the sauce ingredients together in a mixing bowl.
Remove the steaks to a warmed plate and cover to rest - but leave the cooking juices and bits in the pan.
Add all the sauce ingredients to the pan and, stirring constantly, bring to a bubbly simmer.
Plate up the steaks with the sauce spooned over, accompanied by your favourite vegetables and mash.