In a non-stick skillet, sauté bell pepper, onion, and oregano in 2 teaspoons olive oil until soft and slightly browned.
Place in a covered dish to keep warm.
Add remaining oil to pan.
Sauté basa fillets 2 to 3 minutes on each side.
Assemble the sandwich with fillet, sautéed onion and pepper, and top with tartar sauce.