Put 2 tbsp ground flax seeds (or ground chia seeds) to a small bowl, add 5 tablespoons of water and mix with a whisk. Set aside for 5 minutes.
For the pie crust, simply add all ingredients to a bowl and mix with a spatula or your hands until the dough sticks together.
Press the dough into a greased/oiled 8-inch tart or pie dish and up the sides. Put it into the fridge.
Mix the chickpea flour, cornstarch, spices, and water or vegetable broth in a bowl with a whisk. Set aside.
Heat oil in a pan over low/medium heat and add the chopped onion, leek, and bell pepper. Cook the veggies with a lid on for about 15-20 minutes or until lightly caramelized, stirring frequently. Stir in garlic for the last few minutes and preheat the oven to 390 degrees Fahrenheit.
Add the chickpea flour mixture and stir immediately, as it will thicken very quickly. Also, add plant-based milk and tamari.
Let simmer for a few more minutes (the mixture should thicken considerably), then turn off the heat. Pour the filling into the pie crust and smooth it evenly.
You can also add vegan cheese of choice and pan-fried onion rings on top but this step is optional.
Bake in the oven for about 25-30 minutes or until nicely browned.