1. Preheat the oven to 180C/fan 200C/Gas 6. Tip the rhubarb into a large saucepan with 50g of caster sugar and stir-fry for about four minutes, until the fruit just starts to soften. Set aside.
2. Put 85g of caster sugar, almonds, flour, butter, eggs and almond essence in a bowl and beat well with a wooden spoon or electric hand-whisk until smooth.
3. Unravel the pastry, then roll the rectangle a little thinner to create a rough square. Trim the corners a little to neaten. Place onto a large greased baking sheet. Spoon the Bakewell mixture into the centre, then top with the rhubarb, keeping back as much juice as possible.
4. Gather up the edges of the pastry over the filling to make a rough-looking tart. Brush with milk and sprinkle with a spoonful of sugar. Bake for 30 minutes, until golden.