Move an oven rack close to the broiler and place a cast iron pan in the oven. Turn the broiler on high and let the pan heat for 8–10 minutes so it gets very hot.
While the pan heats, drizzle the 1 lb steak with 1 tablespoon oil and season both sides well with salt.
Carefully place the steak into the hot cast iron pan and return it to the oven under the broiler. Broil for 4–5 minutes, flip, and broil another 4–5 minutes, depending on thickness.
Remove the steak from the pan and let it rest for 5 minutes. Slice the steak thinly against the grain so it stays tender for tacos.
To make the salsa, combine 1 diced orange and ½ diced grapefruit in a bowl. Add ¼ cup finely diced red onion, then squeeze in the juice of ½ lime.
Season with ½ teaspoon salt and drizzle in 1–2 teaspoons honey to balance the citrus. Mix gently, then stir in ¼ cup chopped cilantro.
Serve the sliced steak in warm corn tortillas with a spoonful of the citrus salsa.
