Cut core off cabbage and cut into large chunks. Place in food processor.
Remove ends from carrots and cut into large chunks. Add to food processor.
Remove end from leek. Chop in large pieces and add to food processor. Add in cilantro and oregano.
Juice one lime and add to vegetables. Pulse food processor until vegetables are finely chopped - transfer to medium size bowl.
Add mayo and salt and pepper to vegetable mixture. Stir well and refrigerate while you prepare the fish.
Season fish and grill for approximately 4-5 minutes per side.
Warm tortillas, then spread with mayo if desired. Add grilled fish and top with cabbage leek slaw.