For the shortcrust pastry
(or 2 store-bought rolls of shortcrust pastry)
Prepare the shortcrust pastry the day before
Fit your food processor's bowl with the flat beater and pour in 250g of cold butter, 2 large pinches of salt and 500g of flour, and mix until a crumbly texture is obtained
Add 2 beaten eggs and mix gently
Add 1 tbsp of water, mix for a few seconds then gather the dough together with your fingertips and form a ball, then cover with clingfilm
Place in the fridge
Preheat the oven to 190°C
Pour 40g of olive oil into a pan, simmer over medium heat and brown the red onions with 2 tsp of salt until the onions turn clear (about 15-20 minutes)
Add 60g of balsamic vinegar and mix until evaporated
Set aside in a mixing bowl
In a large sauté pan, add 60g of butter and the sliced mushrooms and cook until their jus is rendered
Add 12 chopped garlic cloves and 4 tbsp of chopped tarragon, mix and cook for 2 minutes
Cook on a high heat, add 60g of red wine and mix until evaporated
Chop up the chestnuts, add to the sauté pan then mix
Add the contents of the sauté pan to the red onions in the mixing bowl and let it cool
Heat 20g of olive oil in a pan, add the baby spinach leaves and cook until they soften, drain to remove as much water as possible from the spinach, roughly chop then add to the bowl of garnish
Add 80g of breadcrumbs to the mixing bowl as well as 2 tsp of salt, season with pepper and mix
Divide the shortcrust pastry into three parts and roll out each part to a thickness of 2mm.
With one part, cut a 24cm circle, place it on the removable bottom.
In the second part, cut 2 strips of 7cm high and place them on the edges of the mould, taking care to darken the edges
Pour the cooled filling onto the tart base
Cover with a 24cm diameter shortcrust pastry disc made with the third part and seal the edges well
You can decorate the top of the pie by making a slight incision in the dough with the tip of a knife and by placing a few pastry shapes that you have previously cut out with a biscuit cutter on the top
Using a brush, glaze the top of the pie with the beaten egg
Bake on the bottom shelf of the oven for 45 minutes at 190 degrees.
Remove from the oven and let it cool so that you can cut it sharply and neatly!