Sift the buckwheat and spelt flours into a bowl with 1 tsp salt and scatter over the yeast. Heat the milk and yogurt very gently until tepid, then whisk in the egg yolks. Whisk this mixture into the flour mix and leave for 1 hr to ferment.
Whisk the egg whites to stiff peaks. Stir the beetroot and horseradish purée through the batter, then lightly fold in the egg whites. Leave for a further hour.
Heat the clarified butter in a non-stick frying pan, then add in dessertspoonfuls of blini mix. Fry for about 40 secs, or until they naturally come away from the pan when you shake it. They should puff up, and you’ll need to be careful when turning over as the middle might try and escape a bit – use a palette knife for this. Turn over and fry for another 20-30 secs, then transfer to kitchen paper. Continue with the rest of the mix. Leave to cool, then freeze the blinis for up to one month.
When your blinis are done, top with the soured cream, smoked trout, dill and salmon roe, if using, then serve.