Combine lemon juice, cilantro, garlic, pepper flakes, oregano, salt and oil in a blender. Process to form a thick puree.
Taste and season with pepper flakes and salt as desired. Place 2/3 cup puree in an airtight container and refrigerate; reserve for sauce.
Cut beef into 1 1/2-inch cubes. Place cubes in resealable plastic bag with remaining puree; squeeze out excess air.
Marinate in refrigerator for at least 4 hours or overnight; turn as needed to evenly coat.
Preheat grill to medium-high. Clean it with a stiff brush, then rub with vegetable oil.
Remove beef cubes from marinade, and thread onto skewers; leave a bit of space between cubes. Discard marinade. Sprinkle beef kebabs with salt to taste.
Grill until nicely charred on outside with defined grilling marks, about 2 minutes per side for medium-rare, 3 minutes per side for medium, and 4 minutes per side for medium-well.
Drizzle some reserved chimichurri sauce over kebabs, and serve hot or at room temperature. Serve remaining sauce on side.