Using a knife, mince the cilantro and watercress together. Add it to a bowl, along with the garlic cloves, olive oil, vinegar, diced chile pepper and a few pinches of salt. Alternatively, you can add all of the ingredients to a food processor. Give it a taste and adjust the salt to taste.

Brush a grill or grill pan with olive oil. Set the heat to high heat. Place the slices of halloumi onto the grill and press with a spatula. Grill on each side for 2 to 3 minutes. Flip and cook on the opposite side. Repeat with the remaining slices of halloumi. If you’d like to keep them warm, you can transfer to a plate and cover it with a sheet of foil. Transfer the slices of cheese to a serving plate. Top with a few spoonfuls of chimichurri.