Prepare Watercress Pesto: In a food processor or blender combine watercress, mint, almonds, garlic and the ¼ teaspoon salt. Cover and pulse with several on-off turns to chop the watercress and mint. With the processor or blender running, gradually add water and oil through the feed tube until mixture is well combined, scraping sides of bowl as needed. Set aside.
Sprinkle steaks evenly with the ⅛ teaspoon salt and the pepper. For a gas or charcoal grill, place steaks on the greased grill rack of a covered grill directly over medium heat. Grill, covered, to desired doneness, turning once halfway through grilling. Allow 7 to 9 minutes for medium-rare (145°F) or 10 to 12 minutes for medium (160°F). Serve steaks with Watercress Pesto.
Tip: To toast nuts, spread in a shallow baking pan lined with parchment paper. Bake in a 350°F oven for 5 to 10 minutes or until golden, shaking pan once or twice.