Clean the rhubarb and cut into 3cm lengths, place in a single layer in an ovenproof dish and sprinkle with caster sugar. Roast for about 8-10 minutes at 180C. The rhubarb pieces should be whole but if you pierce with a knife, it should be soft all the way through. Leave the rhubarb to cool.
Meanwhile, toast the oats on a tray under the grill (broiler), watch them carefully so they don't burn, move them around a little so that they toast evenly.
Toast the hazelnuts in the same way or in a dry frying pan which can be easier to manage, let them cool and rub off the skins.
Put the oats, hazelnuts and sugar into a blender and whizz them up, make sure you don't grind them to a powder you want a little bit of texture, you are looking for the texture of rough breadcrumbs.
Whip the double cream and mix with the creme fraiche, honey, and Scotch whisky.
Spoon into individual glasses starting with the oat mixture, the cream, rhubarb and then repeat the layers, finishing with a few of the best pieces of rhubarb and a little spoonful of oat mixture.
You can eat it straight away, but it does improve if you leave it for up to 24 hours, as the oats soak up the moisture from the fruit and cream and soften.