Preheat the oven to 400 degrees. Put the poblanos on a sheet pan and roast, turning occasionally, until charred and blackened on all sides. Remove from the oven. When they are cool enough to handle, peel as much of the skins off as you can. Remove the stem and the seeds and chop. Set aside. Boil the potatoes until tender and drain. Return the potatoes to the same pot. While the potatoes are boiling, melt the butter in a large skillet. Cook the onion, jalapeno, and Fresno peppers until tender. Season with salt and pepper. Add in the garlic and cook for about 30 seconds. Remove from heat. Add the pepper mixture and the cheeses to the potatoes. Add in the sour cream and stir this all together until the cheeses have melted and then serve hot. (This isn't a soft mash. It should be chunky.)