Mix together butter, 2 tablespoon lemon juice, lemon zest, mustard, and capers. Set aside. Prepare a hot fire. Brush tuna steaks with oil. Drizzle remaining 1 tablespoon. lemon juice over fish. Place steaks on a oiled grill set 4 to 15 cm from coals. Grill, turning once, until firm and opaque at center, about 10 to dozen minutes.
Remove from heat and top each steak with 1 tablespoon lemon-caper butter.