Bring 2 inches of water to a boil in a large pot fitted with a steamer basket. Add the shrimp, cover and steam until just pink, 3 to 4 minutes (they'll finish cooking after you remove them from the heat). Spread out on a plate and refrigerate until cold, about 1 hour.
Meanwhile, soak the red onion in ice water for 10 minutes; drain. Whisk the lime juice, pepper paste, ginger, garlic and 1 teaspoon salt in a large bowl until smooth. Peel, devein and roughly chop the shrimp; add to the bowl. Add the red onion, tomato and cilantro and toss. Refrigerate at least 30 minutes and up to 4 hours.