I've done this one from the Feed Zone Cookbook. It's delicious.
The combined taste from the garlic, onion and parsley with the mustard sauce really brings this steak dish alive.
From: Thomas, Biju & Lim, Allen (2011) The Feedzone Cookbook – Fast and Flavorful Food for Athletes Velo Press
Serves 2-4
1-2 pounds (500-1,000g) flatiron steaks*, trimmed of excess fat
olive oil, salt, pepper, and sugar for rubbing the steak
1 tablespoon olive oil
1 teaspoon each minced garlic, onion, and parsley
1 teaspoon whole peppercorns (mixed colour if you have them)
Mustard Sauce
2 tablespoons Dijon or whole-grain mustard
¼ cup yoghurt
1 tablespoon apple cider vinegar
Heat oven to 190°C/375°F. Rub steaks with a bit of oil, salt, pepper and a sprinkle of sugar.
Place a large oven-safe sauté pan over high heat. When it is hot, add the steaks and sear each side. Add 1 tablespoon olive oil, the garlic, onion, and parsley, and the peppercorns to the side of the pan. let cook for a few seconds, until the peppercorns get a nice shine and begin to plump.
Transfer sauté pan to the oven and let cook until the steaks are done to your liking, about 8-10 minutes total. Remove steaks from pan and let rest while you make the sauce.
To make the mustard sauce: Into the hot pan add mustard, yoghurt, and vinegar (this may splatter, so keep a lid handy). Stir; add salt and pepper to taste.
Slice the steaks against the grain. Arrange on plates, then spoon the mustard sauce over the steaks.