02Drain the fish in a colander and return it to the mixing bowl. Add the onion, scallions, tomatillos and half the chiles. Add a good pinch of salt and mix well. Taste and add more chopped chiles if necessary. Stir in the cilantro and add a squeeze of lime.03Line 4 to 6 individual plates with lettuce leaves. Center a tostada or several large tortilla chips on each plate and spoon the ceviche mixture on top.04Surround with lightly salted avocado slices, then add radishes and lime wedges.