Position a rack in the centre of the oven and preheat to 190°C / gas mark 5. Line a baking sheet with greaseproof paper.
Roll the pastry into a rectangle about 1/2cm thick. Cut two (2cm) strips from a long side. Cut the remaining pastry into a 33 by 13cm rectangle and place on the prepared baking sheet. Brush a 2cm border down one long side with water and lay one narrow strip on top, pressing gently. Repeat on the other side. (The filling should be thick enough so you don't need pastry strips on the short sides of the tart to keep the filling from spilling out when the tart is assembled.) Trim any overhang, if necessary.
Very lightly roll a pizza wheel along the inside of the strips to score halfway through the base. Brush the top of the strips with milk, taking care not to drip down the sides. Pierce the base all over with a fork, line it with a piece of aluminium foil that extends along the sides of the strips but not over, and weigh foil down with pie weights or dried beans. Use a paring knife to flute the outer edges of the tart by making small cuts 2.5cm apart.
Bake the tart base until set, about 15 minutes. Remove the foil and weights and bake further until the pastry is light golden brown, about 15 minutes more. Cool on a rack.
Trim the broad tops of the strawberries and put the trimmings in a small saucepan with the rhubarb, sugar and vanilla essence. Simmer over a medium heat, stirring frequently, until the fruit is thick and jamlike, about 15 minutes. Add the butter and puree with an immersion blender or in a food processor until smooth. Cool. Halve the remainder of the strawberries lengthwise.
Spread cooked mixture in the shell. Arrange strawberries in shingled rows down length of the tart.
To make the glaze, stir together the jam and hot water and brush the warm glaze over the strawberries. Dust the sides lightly with icing sugar, slice, and serve with whipped cream.
Cook's Tip: Buy strawberries of roughly the same size so that when they are placed in the tart they fit together nicely.