Heat oil and butter in a medium-sized skillet. Add tuna steaks and fry over medium heat until lightly browned on both sides. Remove from pan and set aside. Cover loosely with foil.
Sauté onions over low heat in same pan (adding more oil and butter if necessary) until softened but not brown. Stir in curry powder and lemon juice. Raise heat to high and deglaze pan with white wine. Simmer,