2 pounds very fresh tuna steak, cut 1-inch thick
4 tablespoons olive oil, plus extra for brushing
2 1/2 teaspoons kosher salt, plus extra for sprinkling
1/2 teaspoon coarsely ground black, plus extra for sprinkling
2 limes, zest grated
1 teaspoon wasabi powder
6 tablespoons freshly squeezed lime juice (3 limes)
2 teaspoons soy sauce
10 dashes hot sauce (recommended: Tabasco)
1 to 2 ripe Hass avocados, medium diced
1/4 cup minced scallions, white and green parts (2 scallions)
1/4 cup red onion, small diced