In a large mortar, mash coriander seeds until they are broken down. Add chile flakes, garlic, and onions, and mash until well-combined. Add thyme, zest, and paprika. Add parsley until herbs release their aromas and essential oils. Stir in vinegar and oil.
Keep at room temperature until ready to serve.
Leftover sauce should be refrigerated and lasts for a few weeks. Bring back to room temperature to serve.
Season the steak with salt and pepper and place on high to medium heated grill and cook on both sides for about 5 minutes until cooked. Take off grill and let rest for 2 minutes. Slice and place over watercress and serve with side of chimichurri.