In a small bowl, combine soy, sherry (or broth), ginger and garlic. Divide the marinade into 2 medium, shallow bowls.
Place the tuna in one bowl and turn to coat.
Place the mango and bell pepper ion the other bowl and turn to coat.
Cover and refrigerate both for 15 minutes.
Coat a grill rack or broiler pan rack with cooking spray. Pre-heat the grill or broiler to medium.
Place the tuna, mango, and bell pepper on the prepared rack or pan. Discard tuna marinade. Grill or broil, basting occasionally with the marinade from the mango bowl, for 4 minutes per side or until the tuna is just opaque and the mango and bell pepper are heated through and glazed.