In a bowl, toss the shredded chicken with the garlic salt, chili powder, cumin, and dried minced onion. Set aside.
In a saucepan over medium heat, add the garlic, green chiles, and chicken broth. Bring just to a gentle bubble.
In a small bowl, whisk together the milk and cornstarch until smooth.
Slowly whisk the milk mixture into the saucepan and stir until combined. Bring the sauce back to a light bubble, then stir in the sour cream. Remove from the heat.
Spray the slow cooker with nonstick cooking spray. Spread a thin layer of sauce across the bottom.
Layer 4 corn tortillas over the sauce, followed by one-third of the chicken, one-third of the cheese, and one-third of the sauce.
Repeat the layers until everything has been used, finishing with the remaining cheese on top.
Cover and cook on LOW for 4 hours.
Garnish with cilantro, jalapeños, scallions, or your favorite toppings before serving.