Add the cilantro, garlic, jalapeno (if using) vinegar, salt and pepper to a blender, and blend until combined, but don't over process.  Whisk in the olive oil. 

Add olive oil to a pan or cast iron skillet and warm over medium-high heat. Salt and pepper both sides of the steak. Once the pan is warm, add steak, and sear to get a good crust (3 - 5 minutes), flip and sear the other side. Continue cooking until desired doneness, then remove from heat and allow to rest for 5 minutes. Cut into thin strips.

In the same pan, add a little more olive oil, and lightly fry the corn tortillas, cooking around 30 seconds a side. You want them to get warm and pliable, but not cook them so long it gets crunchy.

To assemble: pile steak onto warm tortilla, and top it with a drizzle of chimichurri sauce, some cherry tomatoes and a spoonful of sour cream. Throw some chimichurri sauce into a cup on the side for dipping!