HEAT oven to 400°F.
PLACE tortillas in single layer on baking sheet; spray with cooking spray. Sprinkle with 1 tsp. seasoning mix. Bake 7 to 8 min. or until tortillas are crisp and golden brown.
MEANWHILE, combine chicken, remaining seasoning mix and water in large skillet. Bring to boil on medium-high heat; simmer on medium-low heat 5 min. or until chicken is done, stirring occasionally. Add 1-1/2 cups tomatoes; cook until heated through, stirring occasionally. Remove from heat; stir in 1/2 cup cheese.
SPOON chicken mixture onto tortillas; top with remaining cheese, remaining tomatoes, sour cream and cilantro.
serving size = 1 tostada (204 g)