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1
Put oven rack in middle position and preheat oven to 350u00b0F. Toast sesame seeds in a shallow baking pan in oven, stirring occasionally, until golden, 12 to 15 minutes. Cool completely. Transfer 2 tablespoons seeds to a small bowl and 1/4 cup to grinder. Transfer remaining 3/4 cup to a wide shallow bowl.
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2
Halve 3 zucchini crosswise, then cut lengthwise into 3/4-inch shoestrings using slicer. Toss with 1 teaspoon salt in a large sieve set over a bowl and drain 30 minutes.
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3
Squeeze handfuls of zucchini to remove moisture, then roll up in a triple layer of paper towels and squeeze to remove remaining moisture.
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4
Finely chop tuna with a large wet knife (about 3/4-inch pieces). Stir together soy sauce, 2 tablespoons oil, 1/4 teaspoon salt, and 1/4 teaspoon pepper in a bowl, then stir in tuna. Divide tuna mixture into 4 portions. Pack 1 portion in a 1/2-cup measure, then invert onto seeds in shallow bowl. Gently press tuna to form a patty 3 1/2 inches in diameter, then coat completely with seeds and transfer to a wax-paper-lined plate. Form, coat, and transfer 3 more burgers in same manner. Chill, covered with plastic wrap, until ready to cook. Discard any sesame seeds remaining in shallow bowl.
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5
Grind seeds that are already in grinder, pulsing until ground to a flour, then add to remaining seeds (2 tablespoons) in small bowl.
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6
Remove julienne attachment from slicer, then cut remaining zucchini lengthwise into 3/4-inch-thick ribbons using slicer. Gently toss with 1/4 teaspoon salt in a large bowl and set aside (zucchini will wilt and become tender).
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7
Heat 1 inch oil in a 5- to 6-quart heavy pot over moderate heat until thermometer registers 330u00b0F.
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8
Toss all of shoestring zucchini with half of sesame mixture in another large bowl, then fry in 4 batches, turning occasionally with a slotted spoon, until zucchini are golden, about 2 minutes per batch. (Return oil to 330u00b0F between batches.) Transfer zucchini as fried to paper towels to drain. Toss each batch of zucchini immediately with some of remaining sesame mixture.
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9
Heat 3 tablespoons oil in a 12-inch heavy skillet over moderately high heat until hot but not smoking. Cook burgers, turning over once, about 2 minutes total for medium-rare tuna (seeds should be golden brown).
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10
Drain zucchini ribbons and divide among 4 plates, then top with burgers and fried shoestring zucchini. Serve burgers with additional soy sauce on the side.
