Soak the chickpeas overnight in a large bowl of water. You should get about 2 cups of them, when drained.
Rub the beetroot and onion with some olive oil and place them on a baking sheet. Roast in a preheated oven for about 30 minutes until soft, then let them cool. (This can be done well in advance).
Peel the beetroot and onion, then roughly chop their flesh.
Put the beetroot, onion, garlic and drained chickpeas in a food processor and pulse for a few seconds, scraping down the sides of the food processor as necessary. Add the spices, seasoning, then pulse again. When you get a textured mixture, resembling couscous, transfer it to a bowl. Add the chickpea flour and bicarb, then combine well. You should get a thick, sticky mixture. Cover the bowl and chill for 1- 2 hours.
Heat some oil in a small, deep frying pan. Form the batter into small patties or balls and fry 4-5 at a time for about 2-3 minutes on each side, or until golden brown. Add more oil to the pan as necessary.
Transfer the falafels onto a plate, lined with paper kitchen towels.
Serve them warm or at room temperature.