This is a guava. OK, actually, two guavas. They’re small, about large walnut-sized, and sort of pear shaped. Mainly round, but with a slightly pointed stem end. The one on the left is fully ripe (or just about) and the one on the right is less so. There’s a very slight fragrance…sort of fruity/citrus and grassy.
On the inside, you have seeds and firm, salmon/pink flesh. The seeds are hard, and, if you’re making something like guava paste or guava jam, you need to strain them out. They’re as hard as raspberry seeds, but as big as tomato seeds.
Guava can be quite tart. We’re used to it in paste, or otherwise processed with lots of sugar to tone down the tart. I happen to like it straight. It’s almost lemon-like in how tart it is, but the flavor is more rounded, less sharp. When eaten with a spoon out of the skin, it has an almost savory component to it.
In addition to guava paste or jam, which is often used in pastries, you could make it into puree or sauce, which would go well on cheesecake. I think the flavor would play well with chocolate, maybe on a dense chocolate torte. Or I’d bet guava mousse would be nice.