Cut cold butter into cubes and refrigerate till you need to mix it into the flour. Sieve the 2 flours and salt into a mixing bowl, add the oats and whisk it in. Remove the cold butter cubes from the fridge and cut them(or beat with the flat blade of the stand mixie) into the flour till little coarse small crumbs of floury butter are formed. If making the dough by hand, squeeze the butter with your fingers to help it mix with the flour till coarse crumbs are formed.

Add the egg yolk and mix it into the crumbs of flour and butter, so that the mixture starts coming together. If using a processor or mixie, transfer the mixture to a mixing bowl and then proceed as below.

Add ice water, a teaspoon or less at a time and mix till you are able to shape the dough into a ball. Wrap the dough in cling wrap and keep in the fridge for at least 1 hour. I kept mine for 2 hours, but you could do even chill it overnight if you are making the dough a day ahead.

When you are ready to make the tart: Remove the chilled dough from the fridge and place on a flat clean surface or mat on which you have sprinkled a little flour. Roll the dough into a circle, starting at the centre, till it is about 10 inches in diameter and is of even thickness throughout.

While the dough is chilling in the fridge, prepare the filling. Slice the patty pan squash into thin long strips or thin circles. Don't peel the rind of the squash away, it contains most of the nutrients. Dice the green pepper (capsicum) and quarter the cherry tomatoes. If using regular tomatoes, cut them into chunks and use firm but ripe tomatoes which are not very juicy, so that the filling does not make the tart soggy. Wash and pat dry the basil leaves and shred them into strips. Grate the mozzarella cheese and the parmesan (if using)

Pre heat the oven to 230 C (450 F) Transfer the rolled out dough to a baking sheet lined with parchment paper.

Cover the dough with a layer of grated mozzarella cheese, leaving a border of about 2 inches all around. Mix the squash, capsicum and tomatoes with a little salt and pepper (to taste) in a bowl. Cover the cheese with the squash, capsicum and cherry tomato slices, keeping the margin of 2" free. Sprinkle minced garlic and then with olive oil. Sprinkle the grated parmesan if using, and half the basil. (reserve half the basil to garnish the tart after it has baked).

Fold the dough (the 2 inches you had left all around) over the tart from all sides. Beat the egg and teaspoon of water to make an egg wash. Brush the egg wash thinly all over the dough including the sides and the top, without letting the egg drip from the sides.

Place the tart in the oven and bake for 20-30 minutes till the squash is tender and cooked, and the crust is golden. Cool the tart for 10 minutes after removing from the oven, add the reserved basil and then slice for serving.