If you don't care for the sharp peppery flavor of watercress, use 1 1/2 cups of finely chopped fresh spinach. Cracked black pepper is the secret ingredient in this recipe. Substituting ground black pepper for the cracked pepper will make the burgers too spicy hot.
Combine turkey, chopped watercress, 1/4 cup yogurt, 1 teaspoon cracked pepper, salt, and Worcestershire sauce in a bowl. Divide mixture into 4 equal portions, shaping each portion into a 1/2-inch-thick patty. Press remaining 1 teaspoon pepper onto both sides of patties.
Place patties on grill rack coated with cooking spray; cover and grill 5 minutes on each side or until done.
Spread 2 tablespoons yogurt over top half of each bun. Line bottom half of each bun with 1/4 cup trimmed watercress; top each with a patty, a tomato slice, and top half of bun.