Preheat the oven to 220°C/fan 200°C/gas mark 7. Toss the vegetables with the olive oil and season. Place on two large baking trays and roast in the oven for 35 minutes, tossing once halfway through. Reduce the oven temperature to 200°C/fan 180°C/gas mark 6.
Meanwhile, prepare a béchamel sauce. Melt 30g of the butter in a small saucepan over medium heat, and add the flour. Cook out for 1 minute, then gradually add the milk, whisking all the time. Let the mixture come to a boil, then reduce the heat and simmer for a few minutes, until thickened. Add a grating of nutmeg and the sage and season to taste.
For the crumble, whizz together the hazelnuts, oats and parmesan until the nuts are coarsely ground. Tip into a small bowl and rub in the remaining 10g of butter.
Once the vegetables have had 35 minutes of roasting time, tip them into a 2 litre dish, and pour over the béchamel. Toss to combine, then sprinkle over the crumble. Return to the oven for a further 15 minutes. Serve immediately with sage leaves scattered on top.