In a small bowl, combine 2 Tbs. of the olive oil, the rosemary, dry mustard, garlic powder, 2 tsp. salt, and 1 tsp. pepper to make a paste. Rub onto both sides of the steaks.
Position a rack about 4 inches from the broiler and heat the broiler on high. Heat a broiler-safe cast-iron grill pan or skillet on the stovetop over medium-high heat until searingly hot (at least 5 and up to 15 minutes).
Put the steaks in the hot pan and transfer to the broiler. Broil about 2 minutes per side for medium rare (130°F), or until they reach your desired degree of doneness. Transfer to a cutting board, tent with foil, and let rest.
Brush the tops of the onion slices with the remaining 1 tsp. olive oil and season lightly with salt and pepper. Put in the pan, oiled side up, and broil until lightly charred, about 4 minutes. With tongs, separate the onions into rings, toss, and continue broiling until crisp-tender and deeply charred, about 4 minutes more. Slice the steaks if you like, and serve topped with the onions
nutrition information (per serving):
22, Fat Calories
200, Saturated Fat
33, Monounsaturated Fat
4, Polyunsaturated Fat
Photo: Scott Phillips