Preheat your grill or a skillet over medium-high heat. Rub the flank steak with 2 tablespoons of olive oil, then season generously with salt and pepper.
Grill or sear the steak for about 4-5 minutes on each side for medium-rare, or until it reaches your desired doneness. Remove from heat and let it rest for 5 minutes before slicing.
While the steak is resting, prepare the green sauce. In a food processor, combine the parsley, cilantro, garlic, red wine vinegar, red pepper flakes, and 1/4 cup olive oil. Blend until smooth. Season with salt to taste.
Peel and chop the sweet potatoes into chunks. Boil them in a large pot of salted water for about 15-20 minutes, or until tender. Drain and return to the pot.
Add the milk, butter, cinnamon, and salt to the sweet potatoes. Mash until smooth and creamy. Adjust seasoning as needed.
Slice the rested steak against the grain and serve it topped with the green sauce alongside a generous scoop of mashed sweet potatoes.