Step 1Using tongs, roast chiles over stove-top flame, rotating so chiles blacken evenly on all sides, 3 to 5 minutes. (Alternatively, place on pan under broiler and broil a few minutes on each side until blackened.) Place in bowl and cover with plastic wrap about 10 minutes. Remove charred skin, rinse and remove seeds and veins with sharp knife.
Step 2Cook potatoes in boiling salted water until fork-tender, 30 to 45 minutes. When cool enough to handle, peel, slice and place potatoes in buttered 8-inch baking dish.
Step 3Heat oil in skillet over medium heat. Add onions and saute until softened, about 5 minutes.
Step 4Blend chiles, onions and cream in food processor or blender until well mixed and thickened. Season with salt and pepper to taste. Pour chile mixture over potatoes; sprinkle with cheese.
Step 5Bake at 350 degrees until golden brown and bubbly around edges, 30 to 40 minutes.