Pre-heat your oven's broiler.
In a small bowl, combine the ground cumin, salt, and black pepper. Sprinkle evenly over chicken.
Heat oil in a nonstick skillet over medium-high heat. Add chicken to pan and saute for about 3 minutes. In the same pan, add onion, corn, and zucchini. Continue to saute until chicken is cooked through, about 2 more minutes.
Stir in salsa and 2 tablespoons of cilantro. Stirring frequently, cook until the liquid has mostly evaporated.
Arrange two tortillas in a single layer on a baking sheet and lightly coat with cooking spray. Place baking sheet in oven and broil for one minute. Remove from oven.
In the center of each lightly browned tortilla, add about ¾ cup of the chicken and vegetable mixture and top with cheese. Put back in the oven and broil for 2 more minutes, until cheese is melted.
Repeat with remaining two tortillas.
Sprinkle tostadas with remaining cilantro.