Preheat oven to 350°F and lightly spritz a large casserole dish with cooking spray.
Place evaporated milk and taco seasoning in a medium saucepan, whisk to combine, and bring to a low simmer over medium heat.
Once hot, remove the saucepan from the heat, and, while working in 1 cup batches, sprinkle 6 cups of shredded cheese into the hot milk, whisking until the cheese has melted and combined with the milk before adding another cup. The cheese sauce should be thick and just slightly runny. Set aside the remaining 2 cups of cheese to use later.
Place a layer of halved tortillas along the bottom of the prepped casserole dish, overlapping slightly.
Spoon approximately 1/2 cup of cheese sauce over the tortillas, top with a hearty handful of chopped rotisserie chicken, and sprinkle lightly with a couple tablespoons of the reserved shredded cheese. Making sure to spread the ingredients out towards the edges.
Repeat these layers (tortillas, cheese sauce, chicken, and shredded cheese), filling the dish almost to the top, and gently pressing to compact the casserole. Sprinkle the top of the dish with the remaining cheese and place in the oven.
Bake for 25-30 minutes or until bubbly and lightly golden on the top. Remove from the oven and set aside to rest for 10-15 minutes before slicing.
Garnish the casserole with salsa, scallions, avocado, and sour cream (or any of your other favorite Tex-Mex toppings) before serving.