In a small bowl, combine cumin, cayenne, salt, pepper and coriander.
Rub spice mixture generously on both sides of the tuna.
Preheat oven to 350°F for the taco shells.
Heat a nonstick pan on the stove over high heat. When pan is smoking hot, add olive oil, wait 10 seconds, then add tuna. Sear for 1 minute per side, or until fish is cooked on the outside, but rare on the inside. Transfer to a plate.
Separate taco shells and place them in the oven for 4 minutes (timing is everything here, since they go from perfectly done to burned after about 1 additional minute).
While the shells are heating, dice tuna into 1/4-inch cubes.
To serve, fill all eight taco shells halfway with salsa. Spoon tuna over salsa and squeeze a spritz of lime juice overtop. Serve immediately.
Avocado Salsa: Avocado is just ripe if it indents with the firm press of a finger.
Add all ingredients to a medium-size bowl. Toss gently. DO NOT MASH AVOCADO.