In a small bowl, combine the orange zest, brown sugar, garlic, mustard, cumin, basil, salt and pepper. Reserve 1 tablespoon of this mixture for seasoning the white beans. rub the remaining mix on the tuna, coating them well.
In a heavy bottomed sauté pan or a cast-iron pan, warm the olive oil. Increase the heat to high and place the tuna in the pan. Sear for 1 minutes, then turn over carefully, reducing the heat to medium. Sear the other side for 1 more minutes until medium rare. Remove from heat.
NOTE: To grill the tuna steaks - prepare a medium-hot fire for grilling, and brush the grill with oil. Place the grill about three inches above the coals. Grill the tuna for about 3 to 4 minutes on each side for rare, 5 to 6 for medium rare.
In a large saucepan, whisk the orange juice, vinegar, and reserved spices. Bring to a simmer. Add the white beans and basil, tossing well until hot.
Place the beans on a large platter or individual serving plates; drizzle with olive oil. Place tuna steak over the beans and serve.
Makes 4 servings.