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1
Whisk together vinegar, oregano, thyme, garlic, 1/2 teaspoon salt, and 1/4 teaspoon pepper in a small bowl until salt is dissolved. Add 1/3 cup oil in a slow stream, whisking until vinaigrette is combined.
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2
Heat 1 tablespoon oil in a wide 3-quart heavy saucepan over moderate heat until hot but not smoking, then toast couscous, stirring occasionally, until fragrant and pale golden, 3 to 5 minutes. While couscous toasts, stir together broth, water, and saffron in a large glass measure. Add to couscous with 1/2 teaspoon salt and bring to a simmer, uncovered. Simmer, covered, until liquid is absorbed and couscous is al dente, 10 to 12 minutes. Remove from heat and let stand, covered, 10 minutes. Stir vinaigrette to combine, then stir 2 tablespoons into couscous and let stand, uncovered, at room temperature.
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3
Prepare grill for cooking over medium-hot charcoal (moderate heat for gas).
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4
Toss shrimp with 1 tablespoon oil, 1/4 teaspoon salt, and 1/4 teaspoon pepper in a bowl and thread 4 or 5 shrimp onto each skewer (don't crowd, or shrimp won't cook evenly).
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5
Peel onions and trim root ends slightly, leaving ends intact, then halve lengthwise and cut halves lengthwise into 1/2-inch-wide wedges. Insert 1 wooden pick through each wedge to hold layers together while grilling, then put onions in a large bowl with zucchini. Toss vegetables with remaining 2 tablespoons oil, 1/2 teaspoon salt, and 1/4 teaspoon pepper.
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6
Grill shrimp skewers on lightly oiled grill rack, covered only if using a gas grill, turning over once with tongs, until just cooked through, about 4 minutes total. Transfer shrimp, discarding skewers, to a clean bowl and toss with 2 tablespoons vinaigrette.
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7
Grill vegetables on lightly oiled grill rack, covered only if using a gas grill, turning over once, until just tender, about 5 minutes, transferring to bowl as grilled. Remove and discard picks from onions. Drizzle vegetables with remaining vinaigrette and toss to combine.
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8
Spoon couscous onto a large platter or shallow serving bowl. Arrange shrimp and vegetables on top of couscous and sprinkle with feta. Serve warm or at room temperature.
