Roast the chiles over an open flame or under a heated broiler in your oven (Follow instructions here.) Wrap in a towel to sweat for 10-15 minutes, and then peel away charred skin. Rinse, remove stem and seeds, and chop. Set aside.  (Skip this step if using canned chiles)

Prepare the potatoes by washing and slicing. Place the potatoes in a 3qt (3lt) baking dish. Heat your oven to 400°F (204°C).  

Meanwhile, prepare the cheese sauce by melting the butter in a 3 qt. (3lt) saucepan. Sprinkle in the flour, stirring out all the lumps until smooth. Stir in the milk and continue to heat and stir over medium heat for 30 seconds. Add the cheese and the prepared hatch chiles. Stir until all the cheese has melted and the sauce is thick, about 3-4 minutes. Season with cracked black pepper.  

Pour the cheese sauce mixture over the potatoes, and bake uncovered for 50 minutes. Test with a toothpick to make sure the potatoes are tender. If not, allow to bake for 10 more minutes.