This is one of my favorite things in the world - grilled steak, spicy sauce, how can you go wrong? The sauce is actually quite easy to make and is good on a variety of things (you could also serve it on pork, or with pretty much any type of steak).
1 1/2 pounds trimmed flank steak
1 1/2 teaspoons kosher salt
1/2 teaspoon ground cumin
1/2 teaspoon ground coriander
1/4 teaspoon black pepper
3 garlic cloves
1 1/2 cups fresh cilantro
1 1/2 cups fresh flat-leaf parsley
1/4 cup distilled white vinegar
1/3 cup olive oil (sauce) + 1 tbsp for steak
1/4 teaspoon cayenne
Preheat grill to high. Pat steak dry. Drizzle on both sides with the tablespoon of olive oil. Stir together 1 teaspoon salt, cumin, coriander, and pepper in a small bowl and sprinkle mixture onto both sides of steak.
Grill steak on high for 3 minutes per side for medium-rare. Transfer to a cutting board and let stand 5 minutes to rest.
Using a food processor or blender, add cilantro, parsley, vinegar, oil, cayenne, garlic cloves, and remaining 1/2 teaspoon salt, then pulse until herbs are finely chopped. Taste the sauce and add more salt or spice, if necessary.
Slice the steak thinly, along the grain, and serve with the sauce.