Lightly brush tuna with additional oil and season with salt. Crush fennel seeds and peppercorns coarse in a mortar with a pestle and press onto both sides of tuna steaks. Let tuna steaks stand on a plate 10 minutes.
In a heavy non-stick skillet saute fennel bulb in 1 1/2 tablespoons oil over moderately high heat, stirring until golden. Add bell pepper and garlic and saute 1 minute. Add water and simmer vegetables, covered, 10 minutes, or until fennel is tender. Remove lid and, if necessary, boil mixture until liquid is nearly evaporated. Add lemon juice and salt and pepper to taste and keep warm.
While fennel mixture is cooking, brush a heavy skillet, preferably cast iron, with some additional oil and heat over moderately high heat until hot but not smoking. Saute tuna steaks 2 minutes on each side, or until barely pink in center, for medium meat. Transfer tuna steaks to plate and spoon fennel-pepper mixture over and around them.
Recipe courtesy of Gourmet Magazine