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1
Slice one onion into very thin strips.
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2
Combine with the vinegar in a bowl and set aside.
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3
Chop the other onion coarsely and set aside.
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4
In a mixing bowl, whisk the sour cream and milk together.
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5
In a large skillet, pour enough vegetable oil to come 1/4 inch up the sides.
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6
Heat the oil over medium heat until lightly smoking.
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7
Fry the tortillas, 1 or 2 at a time (or as many as will fit in the pan), just until crisp, about 1 minute.
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8
(You may need to add a little more oil for the last few tortillas.)
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9
Drain on paper towels.
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10
Place the stock in a saucepan and bring to a boil.
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11
Add the tomatillos and cook until tender, about 6 minutes; drain, reserving the stock.
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12
Place the tomatillos, jalapenos, chopped onion, garlic, and cilantro in a blender or food processor and blend until smooth.
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13
Season with salt.
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14
In a large skillet over medium heat, heat the vegetable shortening until lightly smoking.
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15
Pour in the tomatillo mixture and stir constantly for about 5 minutes, until thick and dark.
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16
Add the reserved stock and bring to a boil.
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17
Reduce the heat and simmer for 8 to 10 minutes; the sauce should be thick enough to coat the back of a spoon.
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18
Meanwhile, preheat the oven to 350F.
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19
Add the sweet potato to a saucepan of boiling salted water and blanch for 3 minutes, or until tender.
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20
Drain and refresh in ice water to stop the cooking process.
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21
Line an 8-inch square baking pan with 3 tortillas; they will overlap slightly.
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22
Pour one quarter of the tomatillo sauce on top, and pour one quarter of the sour cream-milk mixture on top of the sauce.
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23
Top with one quarter of the cheese, and place a third of the poblanos and sweet potatoes over the cheese.
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24
Repeat this process with the remaining tortillas, sauce, sour cream mixture, and cheese: the top layer will not have poblanos or sweet potato.
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25
Cover the baking pan with foil and bake for 30 minutes, or until the cheese is melted and bubbly.
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26
Remove and top with the drained marinated onions.
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27
Serve with the salsa.
