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1
Rinse chiles, cover with cold water and soak, weighted with a sieve, until completely rehydrated about 8 hours. Do not drain chiles.
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2
Put oven rack in middle position and preheat oven to 350u00b0F.
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3
Remove 1 chile from soaking liquid; working over chile bowl and sieve, cut a slit down one side with scissors and let liquid and seeds drain into sieve. Carefully cut out seedpod, leaving stem intact and letting any loosened seeds fall into sieve, then discard seedpod. Repeat with remaining chiles. Arrange chiles, cut sides up, in 1 layer in a 13- by 9- by 2-inch baking dish.
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4
Reserve 1 cup chile-soaking liquid for sauce. Turn chile seeds out of sieve into a shallow baking pan and spread evenly,toast in oven, stirring occasionally, until dry and a few shades darker, 8 to 10 minutes. Cool seeds in pan on a rack, then finely grind in grinder (makes about 1 tablespoon).
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5
Puree tomatoes, including juice from can, in a blender with reserved soaking water, ground chile seeds, and 1/2 teaspoon salt until smooth, then transfer to a 3-quart heavy saucepan and boil, stirring occasionally, until sauce is thickened slightly and reduced to about 2 cups, 5 to 7 minutes.
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6
Mix beef, chorizo, cheese, and 1/4 teaspoon salt in a bowl, then fill chiles generously with stuffing - slit will not close over it. Pour sauce around (not over) chiles, then cover dish with foil and bake until sauce is bubbling all over, 35 to 45 minutes.
