Step 1

Season the steak on both sides with salt and pepper.

Step 2

Heat the olive oil in a large cast iron skillet over medium-high heat. Add the steak and cook, undisturbed, until browned on the bottom, 5–6 minutes. Flip and cook on the other side for 4 minutes for medium-rare doneness, or until cooked to your liking. Transfer the steak to a plate.

Step 3

Return the skillet to medium heat and add the onion, garlic, and rosemary. Cook, stirring to scrape up any browned bits from the bottom of the pan, until the onion is soft and caramelized, about 6 minutes. Stir in the mustard and water, bring to a simmer, and cook until the sauce thickens, 3–4 minutes. Remove the skillet from the heat and season with salt and pepper.

Step 4

Thinly slice the steak. Spoon the mustard-onion sauce over the meat and garnish with more rosemary leaves. Serve with potatoes or pasta alongside.

Step 5

Enjoy!