Preheat the oven to 375 degrees F. Grease a large baking sheet and set aside. If using fresh plums, wash, slice in half and remove the pits. Set aside. If using frozen, remove them from the freezer and allow to thaw slightly at room temperature while preparing the rest of the recipe.
In a large bowl, mix together the spelt flour, sugar, almonds, cinnamon and sea salt. Cut the butter into pieces and add to the dry ingredients. Using a pastry blender or fork, cut the butter into the flour mixture until it resembles course crumbs. Place in the fridge while making the rest of the cake.
Sift the flour and baking powder together onto a large work surface or baking mat. Create a well in the middle. Pour the sugar into the well and then create another, smaller well in the sugar for the eggs. Crack the eggs into the middle of the sugar. Drizzle the vanilla extract and sprinkle the lemon zest on the sugar around the eggs. Cut the cold butter into small pieces on top of and around the flour.
Using two knives or a pastry blender mix the eggs with the sugar, vanilla extract and lemon zest and then continue to combine with the flour and butter. Once the dough begins to form, use your hands to gently fold and press the dough until it is completely combined and smooth (if the dough is too dry to fully come together and an additional tablespoon of butter at a time and continue to gently fold and press).
On a well-floured surface roll out the dough to approximately the same size as the baking sheet. Fold the dough gently into quarters and place it on the baking sheet. Unfold and press the dough out to the edges of the baking sheet creating a border about half way up the sides of the baking sheet (this helps hold all the plum juices in!). Press together any cracks in the dough that may have formed.
Sprinkle the finely ground almonds evenly on the dough. Place the plum halves flesh side up on top. (Optional: sprinkle extra sugar and cinnamon directly onto the flesh of each plum). Sprinkle the streusel and sliced almonds evenly across the entire baking sheet.
Bake at 375 degrees F for 30 minutes or until the edges of the dough are golden brown. Remove and let cool in the baking sheet for at least 30 minutes before slicing and serving.
This plum cake freezes well and can be refreshed in the oven by places directly under the broiler for 30 seconds to 1 minutes after it has fully thawed.