A tostada is a tortilla that’s fried until crisp, then topped with ingredients of any sort. You can purchase tortillas that are already made into tostadas, but homemade tastes much fresher. We use the oven rather than a skillet to make tostadas; this technique, which requires less oil and allows you to toast more than one at a time. For topping the tostadas, canned tuna combined with sweet corn, smoky chipotle, citrus juice and zest, and quick-pickled onion, plus a little mayonnaise as binder, is a refreshing take on tuna salad.