Combine in a small bowl and stir together the lemon juice, parsley, oregano, chives, salt, mustard, garlic, and red pepper flakes. Brush onto the tuna.
Grill the tuna steaks until lightly browned on the underside, (2-4 minutes). Turn with tongs and grill until browned on the other side (2-4 minutes), more for medium-rare. The steaks should still be pink in the center.
To check, separate the center of the flesh with tongs or make a small slit with a paring knife.