Combine the leaves from 1 bunch watercress, 1/4 bunch parsley, 1/3 bunch cilantro, 1 small garlic clove and 1/2 lemon juiced in a food processor and process until the mixture is smooth. While the processor is running, add 2 tbsp olive oil and blend until the mixture forms a purée. Season with kosher salt and freshly ground black pepper.
Refrigerate until ready to serve with grilled steak.
