Preheat the oven to 180C/350F/Gas 4. Mix the pears, lemon juice and 1½ tablespoons of the caster sugar in a bowl. Spread the pears out on the baking tray, cover with aluminium foil and place in the oven. After 10 minutes, add the apples, cover with the foil again and return to the oven for 30 minutes. Crush the fruit with a fork, set aside to cool then fold in the blackberries.
To make the crumble, place both the flours in a mixing bowl with the butter and brown sugar and rub together with your fingertips until the mixture resembles breadcrumbs. Spread out on a baking tray lined with greaseproof paper and place in the oven for 15 minutes. Remove the tray from the oven and set aside to cool.
Add the remaining three tablespoons of caster sugar and the nuts to a non-stick saucepan and cook over a gentle heat for 5-6 minutes, swirling the pan regularly, until the sugar has melted and coated the nuts evenly and the mixture is golden-brown. Tip the mixture onto a baking tray lined with greaseproof paper and set aside to cool. Once cooled, crush the nuts into a coarse powder.
For the cream, place all the ingredients in a large mixing bowl and whisk to soft peaks, taking care not to over-whisk.
To assemble, spoon two thirds of the crumble into the bottom of the serving glasses. Cover with two thirds of each of the fruit mixture, cream and nuts. Add the remaining crumble followed by the remaining third of the fruit and cream. Scatter over the remaining nuts and serve at once, or chill for a few hours. If you are serving the dish chilled, scatter over the nuts at the last minute.