Bloom the gelatine in water. Bring the cream, sugar and cardemon to the boil and add the gelatine. Pass through a sieve and put into the moulds. Place into the fridge to set.
Make stock syrup with the water, sugar and stem ginger syrup by bringing them all to the boil. The rhubarb should be split into three. With one third, cut into cross sections at a thickness of 1cm then submerge in the stock syrup. The residual heat will cook these. With the other third slice on a mandolin 1mm thick, place into the stock syrup and increase the temperature. Once a little flexible, pull it off, drain and place on a slip mat and cook at 85 degrees for two hours with a reduced fan. Finally with the last third of rhubarb stew it with a little water until soft. Blitz it in a food processor, pass it though a drum sieve and put into a bottle ready for plating.
For the nougatine cook the sugar and water in a pan to make a caramel, once it has reached a golden brown colour remove from the heat and add the almonds. Pour this out onto a slip mat and leave to cool. Toast the oats off in the oven until golden. Break the caramel up and blitz in a food processor (be careful). Mix the nougatine and oats together to make the crumble.
To serve remove the panna cottas from the mould, position the poached rhubarb, rhubarb crisps and dots of rhubarb purée around and place piles of the crumble in between for texture. Finally add the lemon balm to garnish.