In a shallow baking dish, combine rhubarb, strawberries, sugar and extract.
Bake, uncovered, for about 20 minutes, or until rhubarb softens.
Meanwhile, to make crumble: Line an oven tray with baking paper. Combine sifted flour and sugar in a medium bowl; rub in butter until crumbly, stir in nuts. Spoon mixture in a thin layer onto tray. Bake for about 15 minutes, or until mixture is browned lightly. Cool for 10 minutes before crumbling mixture coarsely.
Meanwhile, in a medium saucepan over medium heat, whisk eggs, extra sugar and cornflour for about 2 minutes, or until combined.
Gradually whisk in cream and milk; cook, whisking, for about 3 minutes, or until mixture boils and thickens.
Remove from heat; stir in extra extract.
Pour custard into six serving bowls. Top with rhubarb mixture, then crumble.