How to Make It
Stir together salt, chile powder, pepper, and cumin in a small bowl. Gently toss tuna pieces with half of the salt mixture; set aside remaining salt mixture.
Heat 1 tablespoon of the oil in a 10-inch straight-sided saucepan over high. Add tuna, and cook until browned, about 1 minute per side. Transfer to a plate; set aside.
Heat remaining 1 tablespoon oil in saucepan over medium-high. Add onion; cook, stirring often, until tender, about 5 minutes. Stir in serrano chiles, garlic, and reserved salt mixture. Cook, stirring often, until fragrant, about 30 seconds. Add clam juice, hominy, salsa verde, and lime zest. Bring to a simmer; cook 5 minutes.
Add clams and 2 tablespoons of the cilantro to broth. Cover and simmer until clams open, 6 to 8 minutes. Remove and discard any clams that do not open. Add tuna and lime juice; cook, uncovered, until tuna is heated through, about 1 minute. Divide mixture evenly among bowls; top evenly with radish slices, and sprinkle evenly with remaining 2 tablespoons cilantro.