Start heating up your grill on high. You want it to heat up for at least 15 minutes.
In a high speed blender, add the herbs, oil, vinegar, salt, pepper, and garlic. Blend until smooth, scraping down the sides as needed. If it’s not blending, add equal parts olive oil and vinegar until it blends easily. Transfer to a small-medium bowl. Stir in the radish and avocado.
Pat your steak dry if it’s wet. Generously salt and pepper your steak all over (about 1/2 tsp salt per side). Add a drizzle of olive oil on top of the steak and rub it all over. Grill for about 3-5 minutes per side, or until it reaches an internal temperature of 130-135F for medium rare or 145F for medium. Let rest for 15 minutes, then thinly slice (against the grain).
Arrange the sliced steak on a platter and top with chimichurri down the center. Serve leftover sauce on the side.