The tuna is the small pear shaped fruit which grow on the ends of the paddles (nopales) of the Prickly Pear cactus. The Spanish invaders called them "Figs of Indies". They grow in a variety of colors from bright red, pink, magenta to limey-yellow. They are found through out Mexico, especially in Jalisco and the central states. The small acorn shaped red Tuna found in Oaxaca is used to make tuna nieve (cactus fruit sorbet or ice).
One day I was in Europa, a narrow and long store in San Miguel de Allende that sells liquor, wine and gourmet food items. There were two young, pretty gals wearing sashes over their short dresses that read "Liquor de Tuna" and they were handing out samples. I had to laugh at many of the gringos that curled up their nose of the thought of drinking tuna (fish) liquor!
Ah, viva Mexico.
Several tunas are sweet and can be eaten in many different forms: simply peeled and eaten seeds and all. A refreshing drink is made called Agua fresca de tuna. There are so many different ways to prepare the tunas and each region has their own special recipe. Colonche is a beverage made from fermented tunas. In the state Michoacan, the tunas are skinned and seeds removed before being added raw to a mole de olla. The skin and seeds are removed for a table salsa and for a desert in Hidalgo. In San Luis Potosi, the tunas are used as a relish in salads and in broths and moles in Queretaro.
The tunas can be boiled down to make a dense fruit butter called queso de tuna (prickly pear cheese) and cooked to make a taffy like candy called melcocha. The tuna agria, also known as xoconostle, is acid and somewhat sour is used to lend tartness in some stewed dishes.
If you remember my post on Caramelized Fig Ice Cream (October 18, 2013), you can make the same sauce by substituting the figs with the tunas.
Take care in skinning the tunas, their thorns are as treacherous as the cactus pads. Cut off both ends and then cut the skin off like would would with an orange. Cut them into quarters and remove the seeds. Add them to the caramelized sugar mixture and cook for 15 minutes, stirring often until cooked through. The tunas will remain crisp-tender. They will keep for about a week in the syrup.
Serve over vanilla ice cream. Take note, the flavors are intense so use sparingly.
FYI - The tunas are available in U.S. grocery stores.