Preheat the oven to 450 degrees F. Whisk the mayonnaise, lime juice, chile powder and 1 tablespoon water in a small bowl; cover and refrigerate. Put the tostadas on a baking sheet and bake until crisp, 5 minutes.
Heat a large skillet over medium-high heat; add the raisins and cook, shaking the pan, until plump, about 30 seconds. Add the olives, garlic and bay leaves; cook, stirring, 20 seconds. Add the pico de gallo, olive brine and 2/3 cup water. Simmer until thickened, about 8 minutes.
Add the tuna to the skillet in chunks and cook, spooning the sauce on top, until heated through, about 5 minutes. Sprinkle with the parsley and remove from the heat. Discard the garlic and bay leaves.
Spread about 1 tablespoon of the chile mayonnaise on each tostada. Top with the tuna and sauce, the lettuce and the remaining chile mayonnaise. Serve with lime wedges.
Photograph by Christopher Testani
Courtesy of Food Network Magazine