10 minutes before you begin, soak 150g of dates in boiling water. Preheat the oven to 180°c/350°f, and line the tin with baking paper.
In a large mixing bowl, combine the oats, chopped dates, mixed seeds, baking soda, and salt. Stir until they are combined.
To a blender, add the tahini, coconut oil, vanilla extract, soaked dates, and 60ml/¼ cup of the soaking water. Blend on high until the mix is mostly smooth.
Tip the wet mix into the dry ingredients, then stir them together until the wet mixture completely coats the dry. If it’s a little too sticky, add some extra oats 1 tablespoon at a time. If it's too dry, add a splash of extra date soaking water.
Press the mixture into your lined baking tin, creating an even layer. You can use the bottom of a glass to press the mix firmly into the tin.
Place the tin into the preheated oven for 20-25 minutes or until the date flapjacks are golden brown.
The flapjacks should still be slightly soft when they come out of the oven, but don’t worry, they will firm up as they cool. Once you can handle them, remove from the baking tin and transfer to a wire rack.