1/2 medium pineapple (peeled, cored, and coarsely chopped, about 1 cup)
1/4 cup pineapple juice (divided)
1/4 cup tamari (soy sauce)
3 tablespoons (packed) light brown sugar
1 tablespoon finely grated fresh ginger
1 tablespoon cornstarch
4 tuna steaks, 6 ounces each (3/4-inch thick)
1 tablespoon olive oil
  Salt and freshly ground black pepper