Ingredients

• 1 large bunch leeks (2 to 3 pounds; white and pale green parts only)
• 2 tablespoons unsalted butter
• 1 3/4 cups reduced-sodium chicken broth
• 1 Turkish or 1/2 California bay leaf
• 3 thyme sprigs
• 2 pounds sirloin flap steaks
• 2 tablespoons plus 2 teaspoon olive oil, divided
• 4 Cubanelle peppers (Italian green frying peppers)
• 2 tablespoons red-wine vinegar
• 2 tablespoons Dijon mustard
• 5 tablespoons extra-virgin olive oil
• Equipment: a large (2-burner) grill pan

Instructions

1. Halve leeks lengthwise and cut enough crosswise into 1-inch pieces to measure 8 cups, then wash.
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.
3. Meanwhile, heat grill pan over medium-high heat until hot.
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.
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.
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.
7. Serve steaks over leeks with their juices, drizzle with meat juices from plate, and top with peppers and vinaigrette.