Fermented raspberry leaf.

Black tea is fermented green tea, oolong tea is half-fermented green tea.

And you can change the taste of a few herb teas as well, by fermenting them.
I've done at least raspberry leaf (Rubus idaeus), rowanberry leaf (Sorbus aucuparia) and fireweed willowherb leaf (Epilobium angustifolium), and the taste of each changes a lot.

So you crush the freshly picked leaf, one way or another, in order to get the juice to the surface - a good way is to find a spaghetti machine, a not quite so good way is to make about 10 leaves into a cigar and roll that, hard, with all your weight on your hands, and there are quite a lot of bad ways which aren't even half as practical and fast.

And then you put the crushed-up leaf into a glass jar, put a lid on - not all that tightly, and keep that jar on 50 deg. C in the oven overnight. In the morning you'll find that your leaf has changed color from a light green to a more brownish hue.

Then you dry the leaf, and serve it as tea. I have no idea if the medicinal properties are preserved, but my, it's tasty.

Piece of cake.