2 lb medium leeks (white and pale green parts only)
4 boneless skinless chicken breast halves (1 1/2 lb total)
1/2 tablespoon vegetable oil
2 1/2 tablespoons unsalted butter
1/3 cup dry white wine
1/3 cup chicken broth
1/3 cup heavy cream
Halve leeks lengthwise, then cut crosswise into 1-inch pieces.
Pat chicken dry and season with salt and pepper. Heat oil and 1/2 tablespoon butter in a 10-inch nonstick skillet over moderately high heat until foam subsides, then brown chicken on both sides, about 4 minutes total. Transfer to a plate.
Add wine to skillet and deglaze by boiling over high heat, stirring and scraping up brown bits, until reduced to about 2 tablespoons, 1 to 2 minutes. Add leeks and remaining 2 tablespoons butter and salt and pepper to taste, then simmer over moderately low heat, covered, stirring occasionally, until leeks are wilted, about 10 minutes. Stir in broth. Top leeks with chicken breasts and juices from plate, then gently simmer over low heat, covered, until leeks are tender and chicken is just cooked through, about 8 minutes .
Transfer chicken breasts to a plate. Add cream to skillet and boil over high heat, stirring occasionally, until thickened slightly, 1 to 2 minutes. Add salt and pepper to taste.
Serve chicken on top of leeks.