This is a bael or bael fruit AKA Bengal quince, wood apple, stone apple or seer phael (head-fruit). In Thai it’s known as matum.
In Thailand, bael is usually found in the form of dried slices, which are reconstituted in water to make juice. Our housekeeper brought over a few from her tree and I was surprised at how hard and heavy they were. We did as she said and boiled them, but then accidentally left them out on a hot night and the next day, they had fermented in the shell and burst, oozing a heavy syrup onto our counters. I threw them into the pond out front as an offering to Shiva, although he seems to favor the leaves instead of the fruit.
April, May, and June are mango season here. Everybody who grows them at home brings them into the office or to their friends before the fruit gets too ripe. The coolest thing is that there are over a hundred different species grown and sold here in Thailand. I’ve probably tried about a third of them. To date, the best kind I’ve had are small ones that people grow in their backyards and sell at weekend fresh markets, known generically as mamuang noi (small mango). They have the perfect blend of sweet, tart, and wild flavors, and are at once slightly chewy yet soft.