1. Preheat oven to 350 degrees F. Bake tortillas until golden brown and crisp, about 10-12 minutes. Top with cheese and bake until melts, about 1 minute.
2. Lightly spray a large nonstick skillet with cooking spray and heat over medium-high heat Saute onion with the seasoning until golden. Add the chicken and heat through.
3. To make Tostada, top a corn tortilla with chicken, lettuce, tomato, sour cream.