Mix the flours with the salt and yeast in a large bowl, before adding the milk and the beaten eggs. Mix well, cover and leave for about 1 hour at room temperature or 4hrs+ in the refrigerator until it has doubled in size.
Heat the frying pan over a low to medium heat. Melt the oil and butter in a small pan and brush a little over the surface of the frying pan.
Drop 3 tablespoons of mixture for each blini into the pan, around 7-8cm (3 inches) in diameter. Cook 2-3 at a time making sure they do not touch. Turn when the blini mixture is bubbly and risen across the top, drizzle a little more of the melted butter and oil into the pan and flip to cook the other side. Place the cooked blinis onto a plate and cover whilst you cook the remainder.
Mix the crème fraiche with the mustard and season to taste with salt and pepper.
Cut the beetroot into small diced pieces and shred the mackerel into small pieces.
Place two blinis per person on a plate, spoon some of the mustard crème fraiche on to each blini and top with mackerel and beetroot. Finish with a small handful of your chosen fresh shoots or leaves.
The blinis can be made in a smaller cocktail size by cooking 1 tablespoon of mixture at a time making approximately 36. Blinis freeze well, simply layer in baking paper and plastic wrap before freezin