Preheat oven to 400°F (200°C).
Lightly spray both sides of the corn tortillas with cooking spray and place them on a baking sheet in a single layer.
Bake tortillas for 5 minutes, flip, and bake for an additional 3-5 minutes until crisp and golden brown.
Warm the fat-free refried beans in a small saucepan over medium heat or in a microwave-safe bowl.
Spread 2 tablespoons of warm beans onto each crispy tortilla base.
Distribute the shredded chicken breast evenly over the bean layer on each tostada.
Top each tostada with shredded lettuce, diced tomato, red onion, and fresh cilantro.
Garnish each with a dollop of Greek yogurt, a drizzle of salsa verde, a sprinkle of cotija cheese, and a squeeze of lime juice.