Preheat oven to 170C/150C Fan/Gas 3.
For the poached rhubarb, mix the ingredients together in a saucepan. Boil the mixture until the rhubarb has broken up and most of the liquid has evaporated.
Meanwhile, make the custard. Mix the cream and crème fraîche in a pan and heat until warm. Mix the egg yolks and ginger syrup together in a bowl. Pour in the warmed cream and return the mixture to the pan to warm.
Divide the poached rhubarb between 6 large ramekins then sprinkle half of the stem ginger on top and fill each ramekin with the custard mixture.
Boil a kettle of water. Place the ramekins in a roasting tin, fill the tin with boiling water so the water comes halfway up the sides of the ramekins. Bake for approximately 45 minutes, or until the custard is set but still has a slight wobble.
When cooked, place the ramekins in the refrigerator to cool completely (this will take a few hours, but if you are short of time they can be cooled on a baking tray in the freezer).
For the rhubarb crisps, preheat the oven to 120C/100C Fan/Gas ½.
Put the caster sugar and 75ml/2½fl oz water in a pan and heat until the sugar dissolves. Poach the rhubarb strips in this syrup for 30 seconds. Remove and arrange on a silicon mat on a baking tray. Dry out in the oven for 20 minutes, turning over halfway through. Set aside to cool.
For the topping, preheat the grill to maximum temperature. Top the ramekins with the demerara sugar and place under the grill until the sugar caramelises, alternatively use a chef’s blowtorch. Sprinkle with the remaining chopped stem ginger and the rhubarb crisps. Serve immediately.