1. Peel the sweet potatoes.
2. In a saucepan, cover them with cold water, bring to a boil and cook for 20 minutes. They should be cooked but not too soft.
3. Preheat the oven to 200°C (400°F). Peel and slice the onion, and cut the tomatoes.
4. Heat the sesame oil over low heat, then sauté the onion for 2 minutes until golden.
5. Add the tomato, salt, pepper, and sauté for 3 minutes.
6. Finally, add the tuna and cook for 4 minutes.
7. Cut the sweet potatoes in half, remove the flesh and mix it in a bowl with the tuna filling.
8. Fill the halves with the mixture, sprinkle with sesame seeds, and place them in a baking dish.
9. Bake for 10 minutes. Serve with a green salad.