Oh the wonders of winter citrus! I use fresh lemon and lime juice to “cook” my wild alaskan halibut for some fresh fish tostadas ceviche style. (If you have an electric juicer, this is a minimal energy output way to cook your fish). Since watermelon radishes have shown up at the market, that’s what I’ve been using (sliced paper thin on a mandolin). Use regular raddy’s if that’s what you can find.
If you’re Paleo or avoiding corn, or don’t have tostadas, you can use the leaves of a butter lettuce head to either create the base layer (as in the tostada) or to make a taco-like wrap for the middle.
Feel free to adapt this recipe as you see fit. Experiment with different herbs, cooked fish, chiles, etc. I imagine that if you went spicier on the sauce some firm but ripe papayas might be lovely in the salad part. Make them vegan by subbing the fish for avocado and amp up the veggies. Sky’s the limit!