To prepare the sauce, heat oil in a medium sauce pan over medium heat. Add 1/2 cup onion, cook, stirring until it begins to soften. Stir in garlic and and continue cooking until the onion is translucent. Stir in Chile powder. Add broth, water, oregano and salt. Bring to a boil. Reduce heat to a simmer and cook until reduced by about one third. The sauce should be thick enough to coat a spoon lightly.

Preheat oven to 400?. Coat a 7x11 inch or similar baking dish with cooking spray.

Mash the pinto beans and combine with yoghurt.

Spread about 1/4 cup of the sauce in the baking dish. Arrange 4 tortillas, overlapping to cover the bottom. Top with half the bean mixture, spread thinly. Scatter 2/3 cup cheese and 2 tablespoons onions on top of the beans. If using meat, add here. Top with one third of the remaining sauce, 4 tortillas the remaining bean mixture, 2/3 cup of cheese and remaining onion. Spread half of the remaining sauce on top and cover with remaining 4 tortillas. Top with remaining sauce and cheese.

Bake until hot and bubbly, approximately 15 to 20 minutes. Let stand 5 minutes before serving.