Heat oil in large skillet over medium heat until hot. Add onions; cook about 15 minutes or until onions are golden brown, stirring occasionally.
Add peppercorns, bay leaf and wine. Bring to a boil. Reduce heat to medium-low; simmer 10 minutes, stirring occasionally.
Move onions to side of skillet. If necessary, add additional oil to skillet. Increase heat to high. Add tuna steaks; cook about 3 minutes or until browned on both sides.
Reduce heat to medium; cook 4 to 6 minutes or until fish flakes easily with fork, stirring onions occasionally. Serve tuna steaks with onions.