Step 1: Prepare the syrup by combining 2 cups sugar, 2 cups water, and citrus slices in a pot. Bring to a boil, then simmer for 15 minutes. Stir in lemon juice, remove from heat, and let it cool completely.
Step 2: Preheat your oven to 350°F (175°C) and grease a 9×13 inch baking dish.
Step 3: In a large bowl, beat the eggs and 1/2 cup sugar until light and frothy (about 3-4 minutes).
Step 4: Add the vegetable oil, yogurt, orange zest, and vanilla extract. Whisk until well combined.
Step 5: Gently fold in the semolina, flour, and baking powder until just combined. Do not overmix.
Step 6: Pour the batter into the prepared baking dish and smooth the top.
Step 7: Bake for 30-35 minutes or until a toothpick comes out clean and the top is golden brown.
Step 8: While the cake is still hot from the oven, pour the cooled syrup evenly over the entire surface.
Step 9: Allow the cake to sit for at least 2 hours (or overnight) to absorb the syrup before cutting into squares or diamonds and garnishing with pistachios.