Heat the oven to 200°C/390°F. Wrap the fish fillets in aluminium foil with the butter and half the spices (you might need to do this in two parcels). Mix the veg with the olive oil and the rest of the spices and place on a large sheet pan with the fish parcels. Place in the oven for 15-20 minutes.
Once the fish and vegetables have finished cooking remove the fish from the parcels and flake it. Serve the fish and vegetables with the flour tortillas, fresh coriander, lime wedges and guacamole if desired.