In a food processor, combine the garlic cloves, rosemary, sage and thyme. Process until smooth. Add the olive oil and sherry vinegar to the mixture to make a smooth paste.

Season the skirt steak with the kosher salt and black pepper. Place in a re-sealable plastic food storage bag and add the marinade. Seal the bag and place in a casserole dish. Refrigerate for at least 2 and up to 4 hours, or overnight, turning occasionally.

Preheat a grill to medium-high.

Remove the steak from the refrigerator and allow to come to room temperature for 30 minutes. Brush the excess marinade off the steak and set the steak over the hot grill. Cook for 4 minutes on the first side. Rotate the steak 45 degrees, and cook another 4 minutes. Turn the steak over and continue to cook, about 4 to 6 minutes longer for medium-rare. Transfer the steak to a clean cutting board, and set aside to rest for at least 5 to 7 minutes. When ready to serve, slice the steak across the grain into thin slices. Serve with crusty bread, wilted watercress and salsa verde.

To prepare the watercress, set a large 12-inch skillet over medium-high heat, and melt the butter. Add the minced garlic and cook, stirring, until soft and fragrant, about 1 minute. Add the watercress, 1/4 teaspoon salt, and pinch white pepper, and cook, stirring, until the watercress is wilted and tender, 2 to 3 minutes. Remove from the heat and cover to keep warm.

For the salsa verde, combine parsley, cilantro, garlic, shallots, lemon juice, lime juice, and sherry vinegar in a food processor or mini-chop. Drizzle in the oil to reach desired consistency. Season with salt and pepper.