Lightly frying cooked penne in oil creates a slightly crispy texture that keeps the pasta from becoming mushy in this Mexican-inspired chicken casserole that's made with smoky chipotle sauce and lots of melted cheese.
Place the tomatoes and garlic in a medium (3-quart) saucepan and cover with water. Set over medium-high heat and simmer until the tomatoes are thoroughly cooked and the skins have started to come off, about 10 minutes. Transfer the tomatoes and garlic, along with 1/2 cup of the cooking liquid, to a blender or food processor. Once it has cooled slightly, add the onion, 3/4 teaspoon of salt, 1/4 teaspoon of pepper, adobo sauce, and chipotle chile if using, and puree until smooth.
Sprinkle chicken with oregano, 1/4 teaspoon of salt, and pepper to taste. Heat 4 tablespoons oil in a large, deep and thick casserole or skillet set over medium-high heat. Once hot, sear the chicken thighs about 2 to 3 minutes per side until browned. Remove from skillet, reserve in a bowl.
Pour the remaining 2 tablespoons of oil into the casserole, set over medium-high heat. Once hot, add the pasta. Fry the pasta for a few minutes, stirring constantly, until browned, but not burnt, and it smells toasty. Pour the tomato puree over the pasta. It will jump all over the casserole, so you may want to use the lid as a shield over the casserole. Stir, cover partially and let the sauce cook for about 5 minutes, and keep stirring occasionally, until it darkens and cooks into a thick puree consistency.
Nest the browned chicken thighs in the pasta, pour the chicken broth on top, stir gently, cover tightly, reduce heat to low and cook for 25 minutes.
Preheat the oven to 400 degrees.
Remove the lid, sprinkle the cheese all over the pasta and place the casserole uncovered in the oven. Bake anywhere from 10 to 12 minutes, or until the cheese has completely melted and begun to lightly brown. Serve hot. You may garnish with avocado slices and Mexican crema. Cut into thick slices and serve as you would a cake, since there will be a light crust on the bottom and sides and top.