First, make up the blini batter. Put the flours, baking powder, ½ tsp salt, the milk, egg yolks and melted butter in a bowl and whisk until smooth. In another bowl, whisk the egg whites until they hold stiff peaks. Stir a good spoonful of the whisked whites into the batter to loosen the mixture, then add the remaining egg whites and carefully fold everything together with a big metal spoon.
Heat a small knob of butter and a drizzle of oil in a large frying pan, roughly 23cm wide across the base. Pour a third of the blini mixture in the pan, using your spoon to encourage the batter towards the edge of the pan. Leave to cook for a few mins, until the blini starts to set around the edges and the underside is golden brown. Slide onto a plate, then carefully flip back into the pan and cook for a further 1-2 mins until golden. Repeat with the remaining mixture. Leave the blinis to cool on a wire rack, then place on a plate, separated by a layer of baking parchment and covered with cling film. The blinis will keep like this for 24 hrs.
Mix the crème fraîche, horseradish, most of the lemon zest, a good squeeze of lemon juice and some seasoning in a bowl. In a second bowl, toss the beetroot, oil and a squeeze of lemon juice. Set aside until ready to serve.
Place each blini on a board or large plate. Spoon on dollops of crème fraîche, arrange slices of salmon over and scatter beetroot, capers and the remaining lemon zest on top. Finish with a few rocket leaves and a drizzle of oil. Cut and serve with lemon wedges.