Preheat oven to 400°F Cut slit in end of each chile to allow release of steam during cooking. Brush chiles with oil and roast in oven 5 to 10 minutes, until skins are lightly charred and blistered. Place in plastic bag and seal. Let them steam 5 minutes; using small knife, carefully peel off skin. Slit each chile open from top to bottom and remove seeds.
In boiling salted water cook potatoes until just tender, then drain and place in bowl. Crush with fork, mix in goat cheese, and season to taste with salt and pepper. Stuff chiles with this mixture and arrange single layer in baking dish.
For sauce, in frying pan heat oil, then add red chiles, garlic, and scallions and cook about 2 minutes, until softened. Add tomatoes, corn tortillas, brown sugar, and oregano, and cook over gentle heat 10 minutes. Pour into blender and blitz to almost smooth but slightly textured sauce. Pour sauce over stuffed chiles and bake 10 to 15 minutes. Sprinkle top with grated cheddar cheese, pour sour cream over that, and serve. Makes 4 servings.