Preheat the oven 175C/Fan. Place 175g of the oats in a food processor and pulse until you have a course meal. Now add the remaining oats, ground almond, sugar and butter. Pulse until the butter is incorporated and you have small and large lumps of mixture. Add the water and pulse until the pastry dough just comes together. Tip the whole lot into a bowl. Grease the pastry case with butter. This pastry doesn’t require rolling, you just have to press it into place, smoothing the surface with the back of a spoon.
Line the pastry case with parchment paper and fill with baking beans or rice. Bake for 15 minutes, then remove the baking beans and cook for a further 5-8 minutes or until golden. Cool on a wire rack.
To make the creme patissiere, place the egg yolks in a bowl with the sugar and elderflower cordial, whisk for a minute or two. Now add the cornflour and whisk again until smooth. Pour the milk into a saucepan and bring just to a boil. Remove from the heat and pour it into the egg mixture, whisking as you pour. Now tip the mixture back into the saucepan through a fine sieve. Place back onto the hob on a gentle heat, stirring constantly with a small whisk, scraping the sides down well. Bring to a boil and cook for a couple of minutes, stirring constantly. Pour the thickened mixture into a bowl and cover directly on the surface with a parchment disc to prevent a skin forming. Cool and then place in the fridge to chill.
Once the creme pat is cold, whisk with an electric hand held mixer or a balloon whisk and a strong arm! It will feel quite stiff but persevere, it will become smooth. Whip the cream to soft peak and stir into the creme pat.
To assemble the tart, remove the pastry case from the tin and place on a plate. Tip the elderflower creme pat into the case and level out. Arrange the strawberries on the top and serve.