Preheat oven to 375ºF.
Add butter and oil in a 2-quart saucepan.
Over medium heat, saute onion and rice until onion is translucent and rice is golden, about 5 minutes.
Add the garlic and cook one more minute.
Pour rice mixture into a 8×8 baking dish, cover and set aside.
Wipe the saucepan clean and combine the broth and chiles.
Bring to a boil, then cover and simmer on medium low until chiles are soft, about 15-20 minutes.
Pour the chile mixture into a food processor or blender and add the cilantro.
Process until the mixture is smooth and free of clumps.
Add salt to taste.
Add the chile liquid to the rice, stir and mix well.
Cover baking dish and place in preheated oven.
Bake for 50 minutes; as oven temperatures vary, check for doneness at 40 minutes, and then in 5 minute increments.
Fluff with a fork and serve in a warmed serving dish.