Preheat the oven to 160 C / Gas 2/3 and grease and line a deep 20cm cake tin with baking parchment.
In a large mixing bowl beat together the butter and sugar until pale and fluffy. Add the eggs, one at a time until the mixture is thick and creamy then mix in the vanilla and almond extracts followed by the lemon zest and juice.
Into a separate bowl, sift in the flour then add the baking powder and mix well. Fold the dry ingredients into the wet and beat until smooth. Mix in the rhubarb. Pour the mixture into the prepared cake tin and smooth the top.
For the crumble:
In a bowl, rub the butter into the flour and sugar and mix until it resembles fine breadcrumbs. Mix in the cinnamon and nutmeg.
Sprinkle the crumble mixture over the top of the cake.
Bake in the preheated oven for 90 minutes or until a skewer inserted into the centre comes out clean. Remove from the oven and allow to cool in the tin.
To serve, remove from the tin using the baking parchment to help you, then transfer to a serving plate. Slice and serve.