Position a rack on the bottom of the oven and preheat the oven to 160°C. To make the crust, in a large bowl, combine the flour, sugar and butter. Using your fingers, work the butter into the flour until the mixture resembles coarse breadcrumbs, with a few pea-size pieces of butter remaining. Add the egg, and using a fork, mix until the dough comes together. (Alternatively, place the flour, sugar and butter in a food processor, and using the pulse button, process in short bursts for the same result. Add the egg and process in short bursts again until the dough comes together.) Gather the dough and shape into a rectangle. Wrap the dough in plastic wrap and refrigerate for at least 1 hour or until the dough is firm.
Generously brush a 35cm x 11cm rectangular tart pan with a removable base with softened butter. Roll the dough between two sheets of baking paper until it is 3mm thick. Trim the dough into a 42cm x 18cm rectangle. Freeze the dough for about 5 mins or until firm. Drape the dough over the buttered tart pan. Gently lift and coax the dough into the pan and press the dough into the bottom corners and sides of the pan. Trim the excess and freeze the tart for 10 mins or until firm.
Meanwhile, to make vanilla-rhubarb jam, in a small saucepan, combine the rhubarb, sugar, vanilla and a pinch of sea salt flakes. Bring to a simmer over medium heat, stirring occasionally, for 12 mins or until the jam has thickened and rhubarb has broken down. Stir in the lemon juice. Set aside to cool slightly. Pour jam into a shallow container, cover and refrigerate for 20 mins or until cooled.
To make the almond filling, in a bowl, mix 1 cup of ground almonds and the cornflour. Set aside. In a medium bowl, using a handheld mixer, beat the butter and 2½ tbs of caster sugar on medium speed for about 3 mins or until light and fluffy. Add eggs, one at a time, scraping down the side of the bowl after each addition, until blended. Add the almond mixture and mix until blended. Cover and chill for 20 mins or until slightly firm.
Sprinkle 3 tbs of ground almonds over the bottom of the tart. Smooth the rhubarb jam over the ground almonds. Sprinkle the remaining ground almonds over the jam. Spoon the almond filling over the rhubarb jam and smooth with a small spatula. Sprinkle flaked almonds over the filling. Bake the tart on a large baking tray for 55 mins or until the tart shell is golden and the filling is firm to the touch. Set the tart on a wire rack to cool. Gently lift the tart from the tart pan and transfer to a serving platter.
In a large bowl, toss the strawberries, lemon juice and remaining 2 tbs of caster sugar to coat. Set aside, stirring occasionally, for 30 mins or until the berries have released their juices. Dust the tart with icing sugar and spoon the macerated strawberries across the tart. Using a serrated knife, slice the tart and serve with the strawberries and a dollop of whipped crème fraîche or cream.