Combine parsley, onion, and garlic in a food processor or blender.

Process for 30 seconds until finely chopped.

Add mayonnaise and white vinegar.

Continue processing, scraping down the sides, for 30 seconds or until combined.

While processing, slowly drizzle in olive oil and add red pepper flakes.

Process until blended and the sauce is creamy.

Season with salt and pepper to taste.

Set the creamy chimichurri sauce aside.

Sprinkle the flank steak with salt and pepper.

Grill or broil the steak to your desired doneness, turning once.

Slice the steak against the grain.

Serve the sliced steak immediately with the prepared creamy chimichurri sauce.