For the crust: Place the almond flour and salt in the bowl of a food processor fitted with a steel blade. Drizzle the coconut oil and agave over the top. Pulse until the crust is evenly moistened. If it seems too dry, add a bit more coconut oil. The crumbs should hold together when pressed. Press the crust into the prepared tart pan so that it forms an even layer along the bottom and sides (the crust may not reach all the way to the top of the tart pan). Place in the freezer while you prepare the filling. You can also make the crust 1 day ahead and keep it in the refrigerator, lightly covered with plastic wrap.
For the filling: Place the chocolate in a heatproof bowl set atop a pan of simmering water, ensuring that the bottom of the bowl isn’t touching the water’s surface. Once the chocolate is almost melted, remove the bowl from the pan, pour in the coconut milk, then stir until the chocolate finishes melting and the filling is smooth and glossy. Stir in the vanilla extract and instant espresso.
Immediately pour the filling into the chilled crust, reserving a little to drizzle over the top. Smooth the filling into an even layer. Decorate with berries and sliced almonds, then drizzle the remaining filling over the top. Place in the refrigerator to set completely, about 4 hours or overnight.
When ready to serve, let the tart rest at room temperature for 5 to 10 minutes, then gently remove the tart from the pan. Slice and enjoy.
*To make your own almond flour, place 1 1/2 cups slivered almonds (for a white, finer almond flour) or 1 1/2 cups raw almonds (if you don't mind bits of the skin in the crust and a slightly coarser texture) in a food processor fitted with a steel blade. Pulse until finely ground. Do not over blend or the almonds will turn into almond butter.
Store leftovers in the refrigerator for up to 5 days or freeze for up to 3 months. Let thaw overnight in the refrigerator.