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1
Preheat oven to 350 degrees.
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2
Roast the chiles directly over a gas flame or on a baking sheet 4 inches below the broiler until blacked on all sides (about 5-10 minutes).
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3
Place in covered plastic container, in a plastic bag, or cover with a dish towel and let cool.
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4
Peel and remove seeds.
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5
Slice into 1/4 inch strips.
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6
Heat oil in medium skillet, then add onion and cook, stirring until nicely browned but still a little crunchy, about 5 minutes.
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7
Add garlic and oregano, toss a minute longer, then stir in the chiles and remove from the heat.
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8
Roast the tomatoes on a baking sheet 4 inches below a very hot broiler until blackened on one side (about 6 minutes), then flip and roast the other side.
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9
Cool, then peel, collecting all the juices.
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10
Chop tomatoes coarsely, and combine with juices.
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11
Return chiles to medium high heat, add the tomatoes and their juices, and stir until the juices are reduced, 3-4 minutes.
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12
Season with salt to taste.
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13
Spread half the potato slices in a lightly greased 8x8 inch casserole dish.
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14
Spoon on half the chile tomato mixture, then half the cheese.
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15
Cover with remaining potato slices, chile mixture, and cheese.
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16
Bake until potatoes are tender, 20-30 minutes.
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17
Let stand 10 minutes and serve.
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18
Gratin can be assembled up to one day ahead of time.
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19
Cover and refrigerate until ready to bake.
