Preheat the oven to 425° F. In a large oven-safe skillet, cook the meat and onion, breaking up the meat as it cooks, until the meat is browned, about 5 minutes. Add the taco seasoning and 3/4 cup water. Reduce the heat to medium and simmer until the sauce has thickened slightly around the meat, 5 minutes. Then remove 3/4 of the meat from the skillet to a plate and set aside. To the skillet, add the butter, rice, and poblano. Cook until the rice is toasted, then mix in 1 3/4 cups water and the salsa verde. Bring to a boil over high heat, then reduce the heat to low and cook for 15 minutes.Meanwhile, line the taco shells up in a baking dish. Divide the meat among each shell and top with half the cheese. Bake for 10 minutes, until the cheese has melted.Wiggly the baked tacos into the rice in the skillet. Top with the remaining cheese. Transfer to the oven and bake for 10-15 minutes.Meanwhile, make the ranch. Combine all ingredients in a glass jar and shake or stir until creamy. Serve the hot casserole topped with cilantro, avocado, and ranch. Enjoy!