Chop the onions, the carrot and dice the potatoes.
Dice the poblanos after having taken out all the seeds. Poblano is a gentle petter, so if you want your soup to have more of a kick, you may leave some of the seeds. I personally take them all out: I do like hot dishes, but I prefer the soup to be mild. It is still spicy, but it is a question of preferences.
Preheat the oil and the butter together in a pot until the butter is melted. Add the poblanos, the onions and the carrot. Stir for a few minutes, so that they become well coated with the oil and butter.Add the potatoes and the stock. Cover the pot and bring to boil. When it starts to boil, reduce the heat so that the soup just simmers, and cook for 30 minutes.
After 30 minutes, check that the potatoes are soft: if they are not, cook until they are. Normally 30 minutes is more then enough.
Use an immersion blender to create a smooth soup. At this point the soup will appear liquid. Don’t worry.
Add the yoghurt and/or the sour cream. Stir, and if you do not like the texture, then add more of either yoghurt or sour cream. I like my poblano soup to be creamy, so I may add more than 8oz in total. But it is up to you. I would recommend to start with 6oz, and then add more to your taste. I recommend to use at least 3oz of sour cream, because I find yoghurt to be too bland.
Put the soup in oven proof bowls. Put four tortilla chips in each bowl, and sprinkle some Monterey cheese on top. Put the bowls to broil in the oven. The cheese needs to become golden brown. In my oven it takes less than 10 minutes.