Generously grease a 13” x 9” x 1” jelly roll pan with butter.
Roll out 2/3 of the pastry on a lightly floured board to about 1/4 “thick. Line the tin, trim off any excess, and place into the fridge for 15 minutes.
Roll out the rest of the pastry into a square about 1/4 “thick. Cut out a large heart shape using a template and score small squares on it in horizontal and vertical rows and columns using a metal ruler. Don’t cut all the way through the pastry. Scoop it up onto a floured cake lifter.
Use any leftover pastry to decorate the edge of the pie.
Paint the rows of squares different colours using a thin layer of the colour gel. Place in the fridge until needed.
In a large bowl mix together the fruit, sugar, lemon zest and juice, flour and cinnamon. Spread evenly into the prepared tin. Bake in the oven for 20 minutes. Reduce the heat to 375°F.
Carefully place the heart on top of the fruit. Return the pie to the oven and bake for another 30 minutes until the edges are golden brown.