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preheat a grill to medium high.
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make the marinade:
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whisk the olive oil, garlic, oregano, paprika, 1 teaspoon salt, and pepper to taste in a large bowl.
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transfer 1 tablespoon of the marinade to a small bowl; mix in the yogurt and 2 tablespoons water.
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add the bell pepper and onion to the bowl with the remaining marinade and toss; remove to a plate with a slotted spoon.
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add the steak to the bowl and toss.
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grill the bell pepper, onion and steak, turning once, until lightly charred, about 4 minutes per side for the vegetables and 4 to 8 minutes per side for the steak (depending on the thickness).
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set aside to rest, about 5 minutes.
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meanwhile, grill the pitas until marked, about 1 minute per side.
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slice the steak against the grain.
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fill the pitas with the steak, grilled vegetables and tomato.
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drizzle with the yogurt sauce.