1 cup quartered grape tomatoes [or cherry tomatoes]
3 large radishes, julienned (1/8- x 1/8- x 3-inch strips)
1/4 cup thinly sliced red onion
6 (5-in. ea.) corn tostadas
1/2 cup shredded queso fresco cheese [or Monterey Jack cheese] (about 2 oz.)
Combine lime juice, olive oil and Knorr® Beef flavor Bouillon in small microwave-safe bowl. Microwave at HIGH 30 seconds; reserve 1 tablespoon. Pour remaining marinade over steak in large resealable plastic bag; turn to coat. Close bag and marinate in refrigerator at least 30 minutes.
Meanwhile, combine Mayonnaise with reserved 1 tablespoon marinade in small bowl; set aside.
Combine watercress, tomatoes, radishes and onion in large bowl; set aside.
Remove steak from marinade, discarding marinade. Grill or broil steak, turning once, until desired doneness, about 8 minutes. Thinly slice steak and keep warm.
Evenly divide steak on tostadas. Toss watercress mixture with mayonnaise mixture, then mound on tostadas and sprinkle with cheese.
*SUBSITUTION: To make your own tostadas, fry corn tortillas in small skillet in oil, one at a time, turning once, until crisp; drain on paper towels.
TIP: Top tostadas with your favorite refried black bean recipe.
TIP: Serve, if desired, with salsa verde and chopped avocado.