This is just too good to be so simple. But it's also easy to turn out bad if not cooked right. The secret is in the cooking time. When properly cooked it should resemble a goulash, with about half brown and half yellow coloring. The onions and squash should be slightly caramelized.
The squash has a very subtle flavor that can be overpowered by tomato, so if you add tomato, do not get carried away and use too much.
The squash must be stirred frequently, especially the last few minutes or it will burn. Do not add water, the squash will release all the moisture needed.