For layer 1: In a large frying pan, pan-fry the red onions in the olive oil for about 2 minutes to soften. Stir in the lentils, tamari (or soy sauce), balsamic vinegar and paprika. Cook for a couple more minutes to infuse flavors. Tip onto a plate and set aside.
For layer 3: Using the same pan, add the olive oil with the spinach and garlic, cook for about 1 minute, season with the lemon juice and a pinch of salt. Turn off the heat and set aside.
For assembly: Unroll 1 sheet of the pie dough onto a baking sheet, leaving the dough’s parchment paper underneath. Cut away a 1-inch strip from the short side of the pastry sheet to use to make decoration for the top. Do the same with the second sheet of pastry and set the strips to one side.
Spoon the lentil mixture onto the first pastry sheet, leaving a 1-inch gap around the edge (for sealing later).
Now drain the jarred red peppers and lay them across to form the second layer.
Spoon the spinach layer on top of the roasted red peppers, and spread it out evenly.
Lay the second pie dough sheet over the top.
Crimp in the edges together all the way around to seal.
Cut leaf shapes from the strips of pastry with a knife.
Cut a 1-inch slit in the middle of the pie and arrange the pastry leaves around it.
Brush with the plant-based milk.
Bake for 30 to 35 minutes until the dough is crisp and golden brown.