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1
Halve leeks lengthwise and cut enough crosswise into 1-inch pieces to measure 8 cups, then
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2
Simmer leeks, butter, broth, bay leaf, thyme, 1/2 teaspoon salt, and 1/4 teaspoon pepper in a 12-inch heavy skillet over medium heat, covered, stirring occasionally, until leeks are very tender, about 20 minutes. Uncover and boil until liquid has reduced to about 2 tablespoons. Discard bay leaf and thyme.
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3
Meanwhile, heat grill pan over medium-high heat until hot.
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4
Brush steaks with 2 tablespoons olive oil and season with 1 teaspoon salt and 1/2 teaspoon pepper. Grill steaks, turning once, about 8 minutes total for medium-rare. Transfer to a plate and let rest 15 minutes.
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5
Meanwhile, halve and seed peppers and toss with remaining 2 teaspoon olive oil, 1/4 teaspoon salt, and 1/8 teaspoon pepper. Grill, turning once, until crisp-tender and lightly charred, 6 to 10 minutes total. Cut crosswise into 1-inch pieces.
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6
Whisk together vinegar, mustard, 1/4 teaspoon salt, and 1/8 teaspoon pepper in a medium bowl. Add extra-virgin olive oil in a slow stream, whisking until emulsified. Add peppers.
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7
Serve steaks over leeks with their juices, drizzle with meat juices from plate, and top with peppers and vinaigrette.
