If using a clay tagine, brown the chicken and vegetables in a separate pan, or on the grill. Add the browned ingredients to the tagine and cook following the normal directions. If using a pressure cooker, follow the same browning steps in the pressure cooker pot but only cook the chicken for 15 minutes on medium heat with even pressure. When done, stir in the olives and dates and cook only 2 to 3 minutes longer. Tahini is a Middle-Eastern sesame paste, and is available at most major supermarkets in the ethnic foods aisle, or in specialty Middle Eastern markets or online.