Preheat the oven to 200C/392F.
Line a baking sheet with parchment paper and add the beet slices.
Drizzle with olive oil, season with salt and toss to coat evenly.
Arrange the slices into one layer and roast for 30 minutes.
Remove and set aside to cool.
Crush the walnuts into a mortar with pestle, then transfer into a food processor, together with the parsley, basil, mint, garlic, onion and lemon juice.
Process to incorporate, then add the avocado chunks and pulse to obtain a creamy sauce.
Add the chickpeas, and pulse two or three times to incorporate.
Transfer the mixture into a large bowl, and season with salt and freshly ground black pepper.
Assemble the sandwich - spread 1 tbsp chickpea salad onto a bread slice, then topp it with onion rings, beet chips, and spinach leaves.
Spread 1 tbsp chickpea salad onto another bread slice and close the sandwich.
Serve immediately or pack to-go.