In 2-quart saucepan, combine 2 2/3 cups water, lentils, and 1/2 teaspoon salt; heat to boiling on high. Reduce heat to low; cover and simmer 20 to 25 minutes or until lentils are tender. Drain lentils and return to pan; cover to keep warm.
Meanwhile, evenly season tuna, on both sides, with 1/2 teaspoon salt and 4 teaspoons pepper, pressing in pepper. In 12-inch cast-iron skillet, heat oil on medium-high until hot. Add tuna and cook 5 to 8 minutes for medium or until desired doneness, turning over once. Transfer to plate; cover to keep warm.
Add onions to same skillet and cook 1 minute, stirring. Add broth and capers; heat to boiling. Boil 3 minutes or until liquid is reduced by half. Add lentils; heat through. Remove from heat; stir in lemon juice. Serve tuna over lentils.