Toss the onion with 1 tablespoon kosher salt; set aside until beginning to soften, about 5 minutes. Rinse, drain well and dry thoroughly on paper towels. Toss with the sumac and refrigerate. Halve the avocados and remove the pits. Using a large spoon, carefully scoop the flesh from the skins, making clean halves. Turn each half cut-side down and slice into 6 or 7 wedges.
Lay the avocado wedges on a plate (we like how they look on a long, thin platter). Spoon the onions over the avocado. Season with salt, sumac and cayenne. Drizzle generously with olive oil. Serve immediately.
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