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1
Heat a dry heavy skillet (preferably cast-iron) over moderate heat until hot, then toast chiles, pressing down with tongs and turning occasionally, about 1 minute.
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2
Transfer to a bowl.
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3
Toast garlic in skillet, turning occasionally, until softened and slightly charred, about 5 minutes, then transfer to bowl.
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4
Toast cumin in skillet, stirring, until fragrant and a few shades darker, about 30 seconds, then transfer to a blender along with tomatoes and salt.
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5
Discard stems and seeds from chiles and add to blender.
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6
Peel garlic and add to blender.
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7
Puree until smooth.
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8
Preheat oven to 325F.
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9
Peel potatoes and cut crosswise into 1/8-inch-thick slices.
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10
Cook potatoes in a 2- to 3-quart saucepan of boiling salted water until just tender but not falling apart, about 8 minutes, then drain in a colander.
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11
Arrange slices, overlapping, in oiled baking dish.
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12
Cook onion in oil in a large heavy skillet over moderate heat, stirring, until softened, about 5 minutes.
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13
Add chorizo and cook, stirring, until chorizo is heated through, about 5 minutes.
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14
Spoon mixture evenly over potatoes and top with 1 cup sauce, spreading evenly.
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15
Whisk together eggs, cream, salt, and pepper until combined well and slowly pour into dish.
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16
Bake in middle of oven until custard is just set in center, 35 to 45 minutes.
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17
Let stand 5 to 10 minutes before cutting.
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18
Serve remaining chile sauce on the side.
