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1
Toast the chiles in a dry skillet over medium heat, turning often, until they soften slightly, about 1 minute.
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2
Transfer to a bowl and cover with hot water; let sit 15 minutes.
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3
Transfer the chiles and 3 tablespoons of the soaking water to a blender.
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4
Add 1 1/2 tablespoons lime juice, 1 tablespoon vegetable oil, the garlic, oregano, cumin and 1/4 teaspoon each cayenne and salt; puree until almost smooth.
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5
Toss with the shrimp in a large bowl; refrigerate 15 to 30 minutes.
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6
Meanwhile, heat about 1/4 inch vegetable oil in a medium skillet over medium-high heat.
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7
One at a time, fry the tortillas, turning once, until golden brown and crisp, about 1 minute.
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8
Remove to paper towels to drain.
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9
Halve and pit the avocados; scoop the flesh into a bowl and mash.
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10
Mix in the scallion whites, 1 1/2 tablespoons lime juice, the remaining 1/4 teaspoon cayenne pepper, the cilantro and 1/2 teaspoon salt.
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11
Heat the remaining 2 tablespoons vegetable oil in a large skillet over medium-high heat.
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12
Add the shrimp and cook, stirring occasionally, until just opaque, about 4 minutes.
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13
Season with salt and drizzle with the remaining 1/2 tablespoon lime juice.
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14
Spread the mashed avocado mixture on the tostadas.
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15
Top with the shrimp, some sour cream and diced tomato, and the scallion greens.
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16
Serve with lime wedges.
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17
Photograph by David Malosh
