Preheat oven to 375°F (190°C).
In a bowl, mix mustard with brown sugar, chicken broth and, if desired, allspices.
In a large oven-safe pan, heat the olive oil at medium heat. Brown chicken breasts for 2 to 3 minutes on each side. Brush breasts with a quarter of the mustard sauce. Add the leeks. Salt, pepper, and stir.
Continue to bake for 10 to 12 minutes until the chicken flesh is no longer pink. Remove from the oven and set aside the chicken breasts and leeks on a plate. Cover with aluminum paper.
Pour mustard sauce left in the pan and bring to a boil at medium heat.
Dilute cornstarch in a bit of cold water and add it to the sauce. Boil for 1 minute until it thickens. Serve with chicken and leeks.
For accompaniment: Sweet peas and carrots salad with warm bacon vinaigrette