Start by setting up your BBQ for an indirect cooking session at low temperature (110°C/230°F)
While the BBQ is gaining temperature you rub the slabs with a mixture of equal parts mustard and sriracha and sprinkle the spices on top of them. Slice the onions and spice them with 5 spice powder, fennel powder and sage.
Put the sliced onions on a cast iron baking tray (I recommend cast iron for the full development of flavors)
Place the ribs on top of the onions.
Put the tray in the BBQ for 1.5 hrs. Meanwhile the onions will start to caramelize.
After an hour and a half take a look if the onions are caramelized. If not put the plate on top of the fire until the onions are browned.
Put half of the caramelized onions on top of the ribs and smother them for another hour and a half.