Tuna Tostadas (Spicy Canned Tuna)

Steps

1. Start with the onion, since it needs a quarter of an hour. Peel the red onion, halve it, and slice it into half-moons as thin as you can. Put it in a bowl with the juice of 1 lime, the pinch of sugar, and a pinch of salt, then massage it with your fingers for 30 seconds. Let it marinate for the rest of the recipe: it softens and turns bright pink.
2. Preheat the oven to 400°F (200°C). Brush both sides of the corn tortillas with a thin film of oil and spread them out on two baking sheets without overlapping.
3. Bake for 10 to 12 minutes, flipping them halfway through. If a tortilla puffs up like a balloon, press it down with a spatula. They're done when the edges are golden, the centers are rigid, and they sound hollow when you tap them; they finish crisping as they cool on a rack.
4. While they bake, make the chipotle mayo. Open up the chipotle peppers, remove the seeds if you're wary of the heat, and mince them very finely into a paste. Whisk them with the mayonnaise, 1 tablespoon of the adobo sauce from the can, and the juice of half a lime. It should be smooth and pale orange.
5. Drain the tuna in a strainer, pressing lightly with the back of a fork to push out the oil. Break it into large flakes in a mixing bowl. Chop the cilantro stems and save the leaves for the end. Add the chopped stems and two thirds of the chipotle mayo to the tuna, fold with a fork without mashing, then taste and adjust the salt.
6. Halve and pit the avocados, peel them, and cut them into 1/4 inch (5 mm) slices. Squeeze the juice of the other lime half over them and salt them lightly: the acid keeps them from browning.
7. Peel the mango with a vegetable peeler, cut off the two cheeks on either side of the flat pit, and slice them thinly. Cut the last lime into wedges.
8. Assemble the tostadas at the last moment, right before serving. Spread a thin layer of the remaining chipotle mayo on each tostada, cover with tuna, then fan the avocado and mango slices on top, alternating them.
9. Drain the pickled onion and set a few half-moons on each tostada. Scatter with cilantro leaves and serve right away with the lime wedges, to be eaten by hand within ten minutes while the shells still crunch.