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1
Combine citrus juice, onion, pineapple and jalapeno in a large non-reactive bowl (glass is best).
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2
Cut shrimp into 3-4 chunks each, add to the bowl with the citrus zest and toss well to combine.
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3
Season with salt to taste.
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4
Using the back of a spoon or spatula, press down gently on the mixture so that as much is submerged in the citrus juice as possible.
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5
Cover tightly with plastic wrap and refrigerate for half an hour.
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6
Dice the avocado, finely chop the cilantro and add both to the bowl, mixing gently so as not to bruise the avocado.
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7
Re-cover and refrigerate for another half hour.
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8
Meanwhile, heat an inch of canola oil in a heavy skillet (cast iron is best) over medium heat until hot and shimmering.
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9
Carefully drop in a tortilla and fry on each side until golden brown (about a minute).
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10
Remove using tongs, letting the excess oil drain back into the pan for a few seconds, then drain on a layer of paper towels.
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11
Arrange a handful of shredded lettuce on a plate and place a tostada on top.
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12
Spoon on a generous amount of ceviche, then garnish with more chopped cilantro and a couple of lime wedges and serve immediately.
