Combine, Shallot, Jalapeno, Garlic, Vinegar, and Salt in a medium bowl. Let sit for 10 minutes while chopping the Cilantro, Parsley, and Kale. Once completely chopped add all greens to the bowl with the Olive Oil. Whisk all ingredients together.
In a large cast-iron, or heavy-bottomed skillet heat Butter and Oil over medium-high heat. Season both sides of meat with salt and pepper. Place the meat in the skillet and cook on each side for 4 to 5 minutes. Check meat using a meat thermometer - 125℉ for rare, 130℉ for medium-rare, 135℉ for medium.
Slice steak against the grain and serve with chimichurri sauce.
Flank steak should be served rare to medium in temperature otherwise it may end up too dry or tough.