Sift spelt flour into a bowl, add margarine cut into small flakes, add a pinch of salt and 4 tbsp. cold water and knead. On a well-floured work surface, knead the dough briskly into a smooth ball. Cover and store chilled in the fridge until we finished with the filling.
Wash rhubarb stalks and an apple, remove rhubarb leaves. Cut 300-320 g of rhubarb into small pieces. Core and dice the apple.
In a pot, heat fruit cubes with 1 tbsp. lemon juice, 3 tbsp. maple syrup and a pinch of ground vanilla. Simmer gently for about 7-8 minutes until the ingredients are soft. Then (optional) stir in 1-2 tbsp cane sugar, add 1-2 tbsp brine of pickled beet and dust the fruit pieces with 1-2 tsp flour. Stir in flour and puree ingredients with a hand blender until chunky. Season filling with maple syrup to your taste.
Take the pastry out of the fridge and roll out 2/3 of the pastry on a floured work surface. Grease baking pan with margarine and dust lightly with flour. Pour the dough into the tin and pull up the dough slightly on the edge of the tin.
Pour the rhubarb filling into the mold and spread. Fold the overhanging edge of the dough down on the inside.
Preheat oven to 200°C/392°F top and bottom heat (180°C/356°F convection oven).
Roll out the remaining dough on a lightly floured work surface and cut into strips.
Place the strips crossed over the filling and press lightly on the edge of the dough. Place 1-2 longer strips of dough around the whole cake. Brush the dough strips as well as the dough edge with soy milk (or another vegetable milk) and sprinkle with flaked almonds.
Bake rhubarb pie at 200°C/ 392°F top and bottom heat for 25-30 minutes, until the filling "bubbles" wonderfully and the pie is nice and golden brown (baking time may vary slightly depending on the oven model and age).
Take the rhubarb pie out of the oven, let it sit for 10-15 minutes to cool, remove it from the tin and enjoy it preferably while it is still warm from the oven.