Preheat the oven to 180ºC/Gas Mark 4. For the base, add the walnuts to a blender and blitz until you get a fine meal.
Make the flax egg by mixing the ground flax and water in a bowl, then set aside to thicken. Grease a round loose-bottomed pie tin with oil. Add all the dry ingredients to a large bowl and mix to combine, then add the oil, almond milk and flax egg. Mix thoroughly to form a dough.
Transfer the dough to the tart tin and use your fingers to press into the bottom and up the sides. Poke a few holes with a fork across the bottom, then bake for 4-10 minutes. Remove from the oven.
For the filling, add all the ingredients to a blender and blitz until smooth and creamy.
Layer the filling mix across the bottom of the tart, then top with the asparagus. Press in a little, then add some peas. Drizzle with a little olive oil and a sprinkling of sea salt. Return to the oven and bake for 35 minutes. Allow to cool before removing the pan, then sprinkle on a little fresh mint.