In a small bowl, combine 1/2 cup cilantro, 1/4 cup olive oil, juice of 2 limes, 4 garlic cloves, cumin and serrano chile, stirring until combined. Place flank steak in a large Ziplock bag, and cover with marinade. Seal the bag, then, using your hands, rub the marinade all over the steak. Let sit at room temperature for up to 1 hour (or in the refrigerator for up to 1 day).

While the steak is marinating, prepare the dressing. Add 1/2 cup cilantro, 1/4 cup olive oil, 2 tablespoons lime juice, 1 garlic clove and oregano to a blender, and blend until smooth. Season to taste with salt and pepper. Let sit for 30 minutes before using.

Preheat your broiler to high. Take the steak from the marinade and lay it flat on a rimmed baking sheet. It's ok if bits of cilantro, garlic and chile remain on the steak. Season generously with kosher salt. Broil for 5 minutes on one side, then flip and broil for 4 to 5 minutes more. Timing will depend on personal preference (how rare you like it) and thickness of steak. I like my steak rare. Overcooked flank steak is very tough and chewy.

Let the steak rest for 10 minutes, then slice against the grain.

To serve, place greens in a medium bowl and drizzle with some vinaigrette, tossing to coat. Place dressed greens on a plate, then top with red onion, watermelon radish and slices of flank steak. Finish with an extra drizzle of vinaigrette and a sprinkling of flaky sea salt.