Preheat the grill to a high setting.
Whisk the honey and mustard together in a small bowl.
Place mackerel skin-side down on a tray and brush with the honey mixture.
Grill the fish for 2 minutes on each side then let it rest.
Toast the bread until lightly browned and slice horizontally through the middle.
Cut into 1cm strips and grill the untoasted sides until golden-brown.
Mix avocados, chives, lime juice, and olive oil, then season with salt and pepper.
Pack the avocado mixture into a 7.5cm chef's ring, leaving space at the top.
Spoon whipped cream over the top, level with a knife, and remove the ring.
Scatter with coriander cress and serve with the mackerel and toast.