Thoroughly combine flour, instant sourdough, 50 g yoghurt, egg and 1-2 tablespoons of water. It’s ok if the batter seams a bit tough at this point, but the spinach will add some more liquid to it. Cover and set aside for about an hour.
Meanwhile wash spinach, chop roughly and sautée in a pan for 1 minute. Let cool down a bit. Chop toasted hazelnuts. [You can toast nuts either in a pan - without adding fat - or in the oven. If you smell their intense nutty flavour, they're ready.]
Add remaining ingredients to batter. Heat up your waffle iron.
Dice beet root. Stir in 100 g yoghurt and add salt.
Bake waffles for 3-5 minutes until golden brown and crisp and serve warm with a dollop of beet root yoghurt.