4 cups plain greek style 2% yogurt*
3/4 cup sugar
zest of 2 key limes
1 tablespoon key lime juice
Mix together all of the ingredients until the sugar dissolves. Pour into a ice cream maker and churn until thick, about 30 minutes. Freeze in a freeze safer container or serve immediately for a more "soft serve" style treat.
*I've heard you can make greek-style yogurt-which is thick like sour cream- by straining a double amount of regular yogurt overnight but I haven't tried it. You could also use full fat greek style yogurt here but 2% makes an extremely rich and creamy frozen yogurt and has a fraction of the total fat content.
My thoughts:Due to some quirk in the universe, I often am able to find key limes (at Trader Joe's, Whole Foods or H Mart) for less than the the cost of the more common Persian limes. Normally I just use them as I would a regular lime, but some times their slightly tarter flavor needs a place to shine. I've made a myriad of key lime recipes: the ubiquitous pies (both with and without coconut), sherbet, fruit salads, and even bread and curd but I think one of my favorite things to make with key limes is frozen yogurt. Homemade frozen yogurt is a thousand times better than what you can get at a store: the perfect balance of creamy and tangy and the texture is phenomenal. The tart key limes really bring out the tartness in the yogurt without being over powering. I also think regular limes or even oranges or tangerines would be a good substitute for the key limes if you don't have my luck in finding them.
Note: a couple of people have emailed me about the straining the yogurt option saying that it was a cheaper alternative to the greek yogurt that works just as well. Since I haven't tried it, I don't know if it works just as well. However, I am not entirely convinced that it is cheaper to strain your own. I bought (store brand) greek-style yogurt for $3 for 16 oz, so the total for the recipe was $6. A 32 oz container of regular plain (store brand) yogurt was $3.50. Since you'd have to buy twice the amount of regular yogurt to make faux greek-style yogurt, your total for this recipe would be $7. This means that straining the yogurt yourself is both more expensive and more work. I'd only substitute strained yogurt if I could no longer find the greek-style. Using the greek-style yogurt I went from opening the container to eating frozen yogurt in less than 40 minutes and saved a dollar. Of course, prices might vary depending where you live, but I did do my price comparisons at two national chains.