For the chimichurri: While the steak rests, place the cilantro leaves, onion, remaining 4 cloves garlic, juice of 1 lime, and remaining 1/3 cup vinegar to a blender and puree until combined.
With the blender on low speed, drizzle in the remaining 1/4 cup olive oil in a slow stream. Remove the sauce from the blender and stir in the red pepper flakes and salt, to taste.
For the chipotle cream: Combine the sour cream, chipotle pepper, adobo sauce, juice of 1/2 lime, and pinch of salt.
Place the steak, 3 cloves garlic, 2 tablespoons each olive oil and vinegar, 1 teaspoon salt, and 1/2 teaspoon pepper, and Dijon mustard in a resealable bag. Rub the ingredients into the meat and place in the refrigerator to marinate for 2 hours.
After 2 hours, remove the meat from the bag and pat dry to remove any excess moisture. Lightly season both sides with salt and pepper. Set aside.
Spray an indoor, stovetop grill with nonstick cooking spray and preheat on medium-high heat. Place the steak on the preheated grill and grill about 6 minutes on each side for medium rare. After grilling the steak, cover with tented foil and allow to rest for 10 minutes, before thinly slicing across the grain.
To serve, top each warm tortilla with some mixed greens and sliced steak. Drizzle with the chimichurri and chipotle cream.
Cook's Note: For the steak, I take the time to marinate it in the salt, pepper, Dijon mustard, garlic, olive oil, and red wine vinegar so it's full of delicious flavor on its own. It's a quick process that can be accomplished within a couple of hours prior to grilling and the flavor it imparts to the meat (along with added tenderness) is second to none. The flavors of the marinade also marry the ingredient profile of the chimichurri sauce so there's a nice complement of tastes. The chipotle cream adds a cool heat, along with additional flavor, that brings the dish together nicely.
Servings: 4 to 6 tacos