Sift 1 1/2 tablespoons of chickpea flour into a small container and then add 1 tablespoon of water and stir well until all of the lumps are out.
Add the remaining 1/2 tablespoon of water to the chickpea flour mixture and stir again until it's very smooth. Allow it to sit for at least 10 minutes.
Make the custard base
Sift 2 tablespoons of corn starch into a heavy-bottomed saucepan. (Sifting will help avoid lumps.)
Add 1/3 cup of sugar, 1/8 teaspoon of salt, and slowly pour in 1/4 cup of cold oat milk and stir well with a whisk until all of the lumps are gone and the cornstarch is fully dissolved. Then stir in the remaining 3/4 cup of cold oat milk.
Turn on the heat to medium and stir frequently with a whisk. As it starts to boil slowly, stir constantly.
Once the mixture has boiled slowly for about 30 seconds, slowly add the chickpea flour egg mixture while continuing to stir with the whisk.
Once all of the chickpea "egg" has been added simmer at a low boil for 3 minutes stirring constantly.
After cooking the custard
Remove the custard from the heat and add 2 tablespoons of vegan butter and 1 teaspoon of vanilla extract and stir well until it's incorporated. Then add the zest of 1 lemon if you choose.
Pour the custard into a bowl and lay a sheet of plastic wrap directly over it so that no air is touching it. (This will prevent it from getting a skin on top.)
Chill the custard for a least 4 hours before using it as pastry cream in the tart.
Instructions for the shortbread crust
Preheat the oven to 350° F (176° C).
Mix together 1 cup of flour, 1/2 cup softened vegan butter, and 1/4 cup of sugar until you have a thick smooth dough.
Press the shortbread dough into a smooth even layer on the bottom and up the edges of a tart pan.
Bake the shortbread layer for 30 minutes at 350° F (176° C). Then take it out of the oven and let it cool at room temperature.
How to assemble the fruit tart
Fill the cooled shortbread crust with the chilled vegan pastry cream spreading it out in a smooth even layer.
Top the pastry cream with fruit and berries of your choice. You can make an even pattern or disperse the fruit randomly.
Microwave 1/3 cup of apricot jelly for 30 seconds to warm it up then brush it on top of the fruit for a shiny glaze if desired. (or heat it on the stovetop in a small saucepan)
Serve the tart immediately or place back in the fridge covered with plastic wrap until you are ready to serve.
Notes
Be sure to add a small amount of liquid to both the chickpea egg and the cornstarch mixture and stir well before adding the remaining liquid. This will help you avoid clumps.
Add the chickpea flour egg to the custard slowly while stirring with a whisk. If the mixture is allowed to settle to the bottom, it will harden before it gets incorporated.