1Bring 1 quart salted water to a boil; add 2 tablespoons lime juice. Add prawns and bring back to a boil. Turn off the heat, pour off liquid and cover with a lid. Let prawns steam for 10 minutes off the heat. Spread out on a plate to cool completely.
2Toss prawns with remaining 1/2 cup lime juice, cover and refrigerate for 1 hour.
3Remove prawns from lime juice, reserving a few tablespoons. Combine prawns, onions, cilantro, jalapeño, garlic, cumin, olive oil, salt, pepper and reserved lime juice in a bowl. Transfer to martini glasses or glass bowls and serve with tomatoes, avocado slices and tortilla chips.