Add the mixed berries, banana, Greek yogurt, almond milk, hempseed oil, and honey (if using) to a blender.
Blend the ingredients on high speed until smooth and creamy.
In a small saucepan, combine the hempseed oil, honey, brown sugar, cinnamon, and salt. Heat the mixture over medium heat, stirring constantly, until the sugar has dissolved and the mixture is smooth.
Pour the hempseed oil mixture over the oat and nut mixture and stir until everything is well coated.
Transfer the mixture to the greased baking pan and press it down evenly using a spatula or the back of a spoon.
Bake the muesli bars for 20-25 minutes, or until the edges are golden brown.
Remove the pan from the oven and let the bars cool completely before slicing them into squares or bars.
Store the muesli bars in an airtight container at room temperature for up to one week.