Cut tuna into 1/2-in. pieces. In a medium bowl, mix tuna with onion, lime juice, and soy sauce. Set aside.
In a small bowl, whisk mayonnaise with Sriracha.
Spread half of chips on a large platter or baking sheet. Scatter half of tuna over chips, then top with half of radishes, avocado, jalapeño, and cilantro, then drizzle with half of Sriracha mayo. Repeat with remaining ingredients. Serve with lime wedges.
* Tuna is among the safest of fish to eat raw--it's not prone to parasites--but still, buy from a good market that knows how to keep it at its best.