Soak the raw cashews in hot water for 15 minutes, then drain and rinse.
In a high-speed blender, combine the soaked cashews, soy milk, protein powder, vanilla extract, and maple syrup until the mixture is completely smooth and creamy.
Pour the blended mixture into a small saucepan and whisk in the agar agar powder until no lumps remain.
Bring the mixture to a gentle simmer over medium heat, whisking constantly for 2 to 3 minutes to activate the agar agar.
Divide the custard mixture into two serving bowls or ramekins and refrigerate for at least 1 hour until firm and set.
While the custard sets, place the mixed berries and lemon juice in a small saucepan over medium-low heat.
Simmer the berries for 5 to 8 minutes, mashing them slightly with a fork until they release their juices and thicken.
Stir the chia seeds into the berry mixture, remove from heat, and let it sit for 5 minutes to thicken into a compote.
Once the custard is set, spoon the berry compote over the top and serve chilled.