Place the oil in a large pot set over medium heat. Add the leek, onion, carrot, bell pepper and garlic and cook 5 minutes. Mix in the diced tomatoes, tomato sauce, orange zest and juice, honey, spices and oregano. Bring to a simmer, and simmer the stew, partially covered, for 15 minutes, or until the vegetables are tender. Mix in the tuna and olives and cook a few minutes more, until heated through. Taste the stew, and season with salt and pepper, if needed. Sprinkle servings with parsley.