To make chimichurri, combine cilantro, vinegar, oil, garlic, adobo, oregano, pepper, and chile flakes.
Rinse beef, pat dry, and rub all over with steak seasoning mix.
In a 10- to 12-inch nonstick frying pan over medium-high heat, melt 1 teaspoon butter. Lay steaks in pan and cook, turning as needed, to brown all sides, including edges, about 10 minutes total for very rare (red in center, cut to test). For medium rare, cook 2 or 3 minutes more. To reduce spattering, wipe fat from pan with paper towels.
Transfer steaks to a platter; keep warm. If dark charred bits are present in pan, wipe out with a paper towel. Add remaining butter to pan and stir; when melted, add chimichurri sauce.
Cut meat into 4 portions and set on plates. Pour meat juices from platter into frying pan, then ladle sauce over steaks.
Wine Pairing: A slightly spicy red wine with ripe plums and cherry, such as La Boca 2003 Malbec from Argentina.