Preheat oven to 180°C / 160°C Fan / 350°F / Gas Mark 4. Line a 8-inch / 20cm square cake tin with parchment paper and set aside.
To make the biscuit base, crush the digestive biscuits until they're a fine crumb. Melt the butter and mix the butter and biscuit crumbs together until moistened.
Tip the biscuit crumbs into the lined tin, and press in an even layer across the base of the tin. Bake for 10 minutes until slightly golden and toasted. Set aside to cool.
Now reduce the oven temperature to 170°C / 150°C Fan / 325°F / Gas Mark 3. To make the raspberry swirl, combine the raspberries in a food processor and blitz until pureed. Pass the raspberry puree through a fine mesh sieve that's sat above a bowl/jug. Scrape the underneath of the sieve to get all the maximum amount of puree. Cover this bowl and set aside for later.
To make the cheesecake filling, in a large mixing bowl beat the cream cheese for a minute to soften slightly. Mix the sugar and lemon zest together until fragrant, then add this to the bowl of cream cheese. Beat until combined. Now add the lemon juice, eggs, and salt and mix until combined.
Pour the cheesecake filling over the base, tap the baking tin on the countertop to release any air bubbles. Take spoonfuls of the raspberry puree, and dollop on top of the cheesecake filling. Taking a knife, cocktail stick, or a chopstick (like I do), create swirls.
Bake the cheesecake for 40-50 minutes, until the cheesecake is just set with a slight wobble in the centre. Now turn off the oven, leave the door open and allow the cheesecake to sit in the oven for 10 minutes. Remove from the oven, and leave to cool completely.
Once cool, cover the cheesecake and place in the fridge to set for at least 4 hours, or overnight. Slice up into 16 squares, or bigger/smaller pieces if you desire. The cheesecake bars will keep stored in the fridge for up to 3 days.