Preheat the oven to 180 degrees. Line a small loaf tin with baking paper.

Melt the butter or coconut oil.

Scrub the oranges clean and grate the zest. Make sure not to grate the white pith.

Open the cardamom pods and remove the small seeds. Crush or grind them with the fennel seeds.

Wash the rhubarb. Cut about half into pieces. These are for the top, so choose the best-looking sections. Plan a pattern now so you know how long to cut the pieces and whether to cut them into small rectangles or on the diagonal. Cut the rest into smaller slices to go inside the cake.

Mix the almond flour with the baking powder, salt, psyllium husk, erythritol, cardamom, fennel seeds and orange zest.

Separate the egg whites from the yolks. Set aside 3 whites in a grease-free mixing bowl. Put all the yolks and 2 whites together in a mixing bowl that does not need to be grease-free.

Beat the 3 egg whites until firm. They do not need to be as stiff as for a meringue, but should be thick and airy. Start with the whites because the beaters are still clean and grease-free.

Beat the yolks with the remaining whites until the mixture is airy. It will also increase slightly in volume. Simply use the same beaters for this. Do not reverse the order, as the beaters would become greasy from the yolks and the 3 whites would no longer whip properly to stiff peaks.

Mix the almond flour with the yolk/white mixture to form a thick batter.

Then gently fold the beaten egg whites into the batter using a spatula.

Pour about 1/3 of the batter into the tin. Spread the rhubarb slices over it. Press some a little deeper into the batter and leave some resting on top. Spread the remaining batter over them.

Arrange the best-looking rhubarb pieces on top of the cake. Press them 1 by 1 a little deeper into the batter so that their sides are fully covered. Make sure no parts stick out, as these would burn.

Bake for 55 minutes. The cake is done when a skewer inserted into it comes out clean.

Remove from the oven. Lift the cake out of the tin using the baking paper and place it on a wire rack. Unfold the baking paper. Leave to cool completely.