Preheat the oven to 190ºC (375ºF) and grease and line a 20cm (8 inch) square baking tin with non-stick parchment paper. Leave a bit of paper overhanging for easy removal.
Melt the honey and butter together in a small saucepan over medium-low heat, whisking continuously, just until little bubbles start to pop along the side of the pan.
Remove the pan from the heat and add the vanilla, lemon zest, and Earl Grey tea leaves. Whisk to combine and allow to steep for at least 5 minutes. Once the tea has steeped and the mixture is fragrant, strain the tea leaves from the liquid. Reserve the mixture and discard the used leaves.
Add the cashews to a food processor and pulse until roughly chopped. There should be a mixture of small and medium sized pieces.
Tip the oats, salt, and strained liquid mixture into the processor with the nuts and then pulse 5-6 times until combined. Pulse just until it starts to clump together – don’t over-mix the dough or you’ll lose all of the oats’ texture.
Scrape the dough into the prepared tin and press it down very firmly with your hands or the back of a measuring cup. Bake the flapjacks for 15-18 minutes or until the top is golden brown and the edges are a bit darker and slightly crispy. There will be a slight softness to the middle of the bars and that’s okay.
Leave the flapjacks in the tin on a wire rack to cool completely before lifting them out and slicing. Enjoy with a drizzle of melted white chocolate, if desired.