Step 1 Line an 18cm pie tin with pastry sheet, trimming to fit, then place in freezer to chill for 20 minutes.
Step 2 Meanwhile, place a medium saucepan (which has a lid) over high heat. Add rhubarb, apples, half the sugar and 1 tablespoon water; bring to the boil. Reduce heat to medium–low and cover pan; simmer for 5 minutes, or until rhubarb softens.
Step 3 Place cornflour in a small bowl with 1 tablespoon cold water and stir until smooth; add to rhubarb saucepan with strawberries and stir to combine. Return mixture to the boil and cook, stirring, for 1 minute, or until thick. Remove pan from heat and leave to cool.
Step 4 Preheat oven to 180°C. Place pie tin on a large baking tray. Line pastry with baking paper; top with ceramic pie weights (or uncooked rice). Bake pastry for 12 minutes, then remove weights and paper; bake for 7 more minutes, or until golden, then leave to cool slightly. Reduce temperature to 160°C.
Step 5 Combine remaining sugar with oats and cinnamon in a medium bowl; rub in table spread until mixture becomes a crumble.
Step 6 Spoon rhubarb mixture into cooked pastry base and top with crumble; bake for 20 minutes, or until golden. Leave crumble to cool for 20 minutes, then slice and serve with vanilla custard.