Preheat oven to 325F. Make the filling: combine 12 oz. blackberries with lemon juice, honey, vanilla extract, and sea salt. Mash blackberries with a fork (watch for splatters!) and mix well.
Add remaining 6 oz. blackberries (do not mash), stir to combine, and set aside. Make the crust & crumble: combine almond flour, tapioca starch, lemon zest, sea salt, and ground cinnamon in the bowl of a large food processor. Pulse to combine.
Add butter, honey, and egg. Pulse again to combine. Dough will be crumbly; do not over-mix. Assemble: move 2/3 of the dough to an 8x8” baking pan. Press into an even layer.
Top with blackberry filling.
Add ½ c. sliced almonds to the remaining 1/3 dough remaining in the food processor. Pulse briefly to incorporate. Crumble over the blackberry filling in large chunks.
Do not press in.
Bake for 30-35 minutes, or until golden brown.