Remove steak from fridge about an hour before you want to eat. Steak will cook more evenly when it is at room temperature. Season it generously on both sides with sea salt and pepper.
Combine cilantro, ⅓ cup olive oil, juice from 1 lime, 1 garlic clove and ¾ teaspoon sea salt in a blender and blend until smooth, stopping to push cilantro down into the blades, as necessary. Set aside.
Heat 1 tablespoon cooking oil in a large skillet over medium heat. Add sliced onion and cook 5 minutes, until it’s golden brown but still has some crunch to it. Add 2 minced garlic cloves, red pepper flakes and a big pinch of sea salt; cook for 1 minute, stirring frequently. Add broth or water and chard. Reduce heat to medium-low, cover the pan and cook for approximately 5 minutes. Uncover the skillet, raise the heat back to medium-high and cook until the water/broth is evaporated, stirring occasionally. Season with sea salt to taste. Set aside.
Heat 1 tablespoon cooking oil in a heavy cast iron pan over medium-high heat. Sear steak on each side for three to five minutes. Remove from pan and let rest for five minutes. Wipe the pan clean and warm the tortillas. Slice the steak thinly against the grain. Divide chard between tortillas. Add sliced steak to tortillas and top with cilantro sauce and sliced radishes.