Toss the potato slices in 1tbsp of the oil, and lay them in a single layer on a baking sheet. Season generously with salt and black pepper. Bake at 200°C (Gas Mark 6 / 400°F) for around half an hour (turning once halfway), until soft inside, and crispy and golden brown outside.
Meanwhile, add the walnuts to a mini food processor along with the remaining 1tbsp of oil and all of the spices. Blitz for a few seconds until the mixture resembles breadcrumbs.
When the potatoes are crispy, allow them to cool for a couple of minutes and then add your toppings - sour cream, salsa, chopped spring onions, and the walnut taco 'meat'.