1 Rinse, core and quarter tomatoes. Remove seeds and chop finely. Rinse and thinly slice scallions. Rinse and chop jalapeños. Rinse cilantro, shake dry, remove stems and chop leaves coarsely.
2 Halve and juice lime. In a large bowl, mix together all prepped ingredients along with 1 tablespoon olive oil and 2 tablespoons lime juice. Season with salt and pepper.
3 In another bowl, mix chili powder and yogurt. Season with salt and a little lime juice. Fill piping bag with mixture.
4 Grind coriander seeds in a mortar. Rinse tuna, pat dry and cut into 2 equal-sized rectangular pieces. Season with salt, pepper and crushed coriander.
5 Heat the remaining oil in a non-stick pan over high heat. Sear tuna, 20-30 seconds per side. Set aside and let cool slightly.
6 Meanwhile, rinse lettuce, shake dry and cut into thin strips. Divide lettuce among taco shells.
7 Halve avocado and remove pit. Scoop out flesh and cut into strips. Divide avocado slices and prepared tomato mixture over the lettuce.
8 Cut tuna pieces into slices and divide among taco shells. Top with chili-yogurt sauce and serve the tuna tacos immediately. Use any extra chili-yogurt sauce as a dip.