I finally figured out what I have been eating for 2 weeks in Rwanda. It is quite popular in the markets and restaurants there, along with passion fruit. I have included some passion fruits at the top of the photo, for comparison. People often get the two mixed up.
It has a distinctive and complex flavour, tasting sour but flesh. It can be made into a juice, or served as fruit after a meal. I have asked many people, but no one has been able to tell me its name in English. Below is the cross section of the tree tomato. A passion fruit is included in the upper left for comparison.
It turns out to be a tree tomato, also known as tamarillo, or tamamoro. It is high in vitamins and iron.