Preheat the oven to 350 degrees.

Combine the oats, the dates and 1/8 teaspoon salt in a food processor until the mixture begins to stick together.

Press the mixture into a pie pan and bake for 10 minutes.

Add the Japanese sweet potato, the yogurt, the vanilla and a pinch of salt to a blender.

In a small pan, dissolve the agar powder into the water.

Heat the agar mixture over medium heat until it boils. Then, turn it down and continue stirring it over low heat until it thickens (no more than one minute).

Add the agar mixture to the blender and blend until smooth.

Put about 1/3 of the mixture in the prepared pie crust.

Add the raspberries to the mixture remaining in the blender, and blend until smooth.

Spread 1/2 of the mixture on top of the first layer in the crust.

Add the date sugar and the blueberries to the remaining mixture in the blender, and blend until smooth.

Spread the mixture on top of the other layers in the crust.

Garnish the top with fresh berries or extra oat and date crumble, and refrigerate for at least 4 hours.