Place the extra virgin olive oil, chopped cilantro, pineapple chunks (or juice), lime juice, ½ teaspoon salt, and ¼ teaspoon black pepper in the blender. Blend until smooth, thick, and pourable, adding more oil or citrus juice (if needed) to reach your desired flavor and consistency. Set aside.
Heat the grill pan over medium/high heat while preparing the flank steak. Slice the flank steak across the grain into ¼" thick slices and season the strips with the remaining salt and pepper on both sides.
Thread the sliced flank steak onto the prepared skewers, making sure to flatten the beef on the skewers so as much meat as possible so the most meat touches the hot grill surface.
Working in batches (depending on the size of your grill pan) if needed, brush the hot grill pan with avocado oil. Lay the skewers perpendicular to the grill grates. Cook for 2 - 3 minutes making sure to brush the meat with additional avocado oil to prevent sticking. Flip the skewers and continue cooking the second side for an additional 2 - 3 minutes or until the flank steak is cooked to your preferred doneness.
Remove to a platter and drizzle the skewers with the cilantro chimichurri and top with the (drained) pickled red onions.
Notes
Store any leftover cooked flank steak in an airtight container in the refrigerator for up to 5 days (skewers removed)Flank steak skewers may be frozen after cooking. Remove the skewers and place the cooled, cooked meat in an airtight freezer container. Freeze for up to 2 months.Store leftover cilantro chimichurri in an airtight container in the refrigerator for up to 3 days. May be frozen. Freeze in ice cube trays. Once frozen remove the cubes and store them in an airtight freezer bag for up to 3 months. May be used one or more cubes at a time. Refreeze unused cubes.