Combine the sauce ingredients, and set aside. Put the leek and pepper in a large skillet with a little bit of olive oil. Saute for about 3 minutes, just until the vegetables begin to wilt. Remove vegetables from the skillet and set aside. Add chicken to the skillet (and more olive oil, if necessary), and cook in the vegetable juices until no longer pink. Add the sauce to the skillet. Bring to a boil, and simmer on low heat for about 5 minutes. Prick the chicken with a fork, to let the juices seep in. Add the vegetables back into the skillet and cook for an additional 5 minutes, or until the chicken is cooked through. While the chicken is simmering, bring a large pot of water to a boil over high heat. Dissolve the bouillon cube in the water. Add the spaghetti, and cook to desired doneness. Transfer the pasta to a serving bowl. Arrange the chicken and vegetables and sauce on top, and enjoy!