Preheat oven to 180 celsius (350 Fahrenheit) and wrap each beetroot in foil, along with a teaspoon of water. Place on a baking tray and roasted for 40 to 60 minutes, or until the beetroot is tender. Wrap the garlic (leave the garlic in its paper skins) in foil and add for the last 10 minutes of cooking, and cook until soft and golden. Leave the beetroot to cool before peeling.
Place a frying pan over a medium low heat and add the cumin and coriander seeds and cook until fragrant. Place the spices in a coffee grinder, or mortar and pestle and grind until a powder.
Roughly chop the beetroot and place in the bowl of a food processor. Squeeze the garlic from it's skin and add to the food processor along with all the remaining ingredients, except the pistachios and olive oil. Process until smooth. Check the seasoning, and adjust as necessary.
Serve with a drizzle of olive oil and the chopped pistachios, if using.
For a vegan version, simply substitute the yogurt for a dairy-free yogurt like soy or coconut.