Preheat the oven to 200 degrees celsius / 400 fahrenheit with a rack positioned in the middle of the oven.
Whisk the dry ingredients together in a large bowl until well incorporated. In a small bowl, whisk together the ground flaxseed & the 6 Tbsp of milk or water until combined. Set aside & allow to thicken. In a measuring jug, mix together the 1 1/2 cups almond milk & the apple cider vinegar. Set aside for 5 — 10 minutes to thicken slightly & activate (resulting in 'buttermilk').
In a medium mixing bowl, whisk together the flax mixture, melted coconut oil & coconut yoghurt. Slowly whisk in the almond 'buttermilk', followed by the rest of the wet ingredients. Pour the wet ingredients over the flour mixture and gently fold until just combined. A few floury patches are ok, just don't overmix the batter or you will end up with tough & flat muffins.
Spoon the muffin mixture into lined muffin tins, filling to about 3/4 full. Top each muffin with a thin slice of blood orange & scatter with a little sugar so that the orange caramelises a little in the oven.
Bake in the preheated oven for about 25 — 30 minutes (less if you are making smaller muffins), until golden brown on the edges & a skewer inserted into the middle comes out clean. Allow to cool for 10 minutes in the muffin tin & then remove the muffins & allow them to cool on a wire rack.