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In a large bowl mix together the pomegranate syrup and pomegranate juice.
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Wash the apples and pat dry. Peel, quarter and core the apples. Cut each quarter into thin slices.
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Place the apple slices in the pomegranate syrup/juice mixture. Make sure they are completely submerged in the liquid. If too many pieces are floating on the top of the liquid, place a small dish on top of the floating apples to keep the apple slices under the liquid.
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Cover the bowl with plastic cling film and place in the refrigerator for 24 hours.
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Chop the pistachio nuts into small pieces.
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Place the rolled oats, pistachio nuts, raw sugar, and 1 stick plus two tablespoons of the unsalted butter in a bowl and use your hands to rub the mixture together until it forms streusel crumbs. You can also use a fork to mix the streusel.
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Cover the bowl with plastic cling film and place in the refrigerator.
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In a separate bowl, whisk together the all-purpose flour, baking powder, and salt. Set aside.
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In a mixer, cream together the rest of the butter (2 sticks or 1 cup) together with the white granulated sugar until creamy.
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Stop the mixer and scrape down the edges of the bowl with a rubber spatula. Add the vanilla and continue to beat the mixture on medium-high speed for another 5 minutes, until the mixture is creamy and fluffy.
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Add the eggs one at a time one after the other, making sure each is fully incorporated before adding the next.
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Turn the mixer down to low and slowly add the dry ingredients.
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Stop the mixer when the dry ingredients have been fully incorporated into the batter.
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Strain the apples from the pomegranate liquid. Reserve the liquid for later use or discard.
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Preheat the oven to 350 degrees F.
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Line a 11 x 12 baking sheet with baking or parchment paper and grease the paper.
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Spread the batter evenly out onto the baking sheet.
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Place the apple slices on top of the batter. Make sure you lay them all in one direction and in rows as neat as you can.
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Remove the pistachio streusel from the refrigerator and crumble it evenly over the top of the apple pieces.
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Place the baking sheet in the oven. Bake for 40 minutes until golden brown.
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Remove the baking sheet from the oven. Let the cake rest for 10 minutes and then transfer the cake to a cooling rack.
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When cake has cooled completely, cut into pieces, place on a decorative plate and serve.
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If desired, scatter additional chopped raw pistachio nuts over the top for garnish before serving.