To make the sauce: Put all ingredients for the sauce into saucepan. Mix well with a whisk until well combined. Cook over medium heat, stirring occasionally, until thickened, about 5 minutes. Taste and add more chile powder if desired. Set aside.
To assemble the casserole. Preheat oven to 350F. Put beans, green onions, corn, olives and green chiles, into large bowl and mix gently until well combined.
Spread 1.5 cups of sauce over the bottom of a oblong nonstick baking dish. Place 3 to 4 corn tortillas in the bottom of the dish. Spread half of the bean misture evenly over the tortillas. Place another 3 to 4 tortillas over the bean mixture, then spread the remaining bean mixture ov the top. Cover with 3 to 4 more tortillas, and the pour the remaining sauce over the tortillas. Cover the baking dish with parchment paper, then cover with aluminum foli, crimping the edges over the baking dish.
Bake for 45 minutes.
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