Preheat your oven to 350°F. Line a 9x13-inch baking dish with parchment paper, leaving an overhang on the sides for easy removal.
In a medium mixing bowl, cream together the softened 1 cup unsalted butter and 1/2 cup granulated sugar until light and fluffy.
1 cup unsalted butter, 1/2 cup granulated sugar
Add the 2 cups all-purpose flour, 2 tbsp poppy seeds, and 1/4 teaspoon salt, mixing until well combined and a dough forms.
2 cups all-purpose flour, 2 tbsp poppy seeds, 1/4 teaspoon salt
Press the dough evenly into the bottom of the prepared baking dish.
Bake the crust in the preheated oven for 18-20 minutes, or until lightly golden. Remove from the oven and set aside to cool slightly.
In a large mixing bowl, whisk together the 1 1/2 cups granulated sugar and 1/4 cup all-purpose flour until well combined.
1 1/2 cups granulated sugar, 1/4 cup all-purpose flour
Add the 4 large eggs, 2 tbsp lemon juice , 3 tbsp orange juice, 3 tbsp blood orange juice, 3 tbsp grapefruit juice, and 1 tablespoon lemon zest. Whisk until the mixture is smooth and well blended.
4 large eggs, 2 tbsp lemon juice , 3 tbsp orange juice, 3 tbsp blood orange juice, 3 tbsp grapefruit juice, 1 tablespoon lemon zest
Pour the citrus filling over the slightly cooled crust.
Return the baking dish to the oven and bake for 20-25 minutes, or until the filling is set and no longer jiggles in the center.
Remove from the oven and let the citrus bars cool completely in the baking dish.
Thinly slice the 2 orange slices, 4 lemon slices, 2 grapefruit slices, and 4 blood orange slices. Arrange the slices decoratively on top of the cooled citrus bars.
2 orange slices, 4 lemon slices, 2 grapefruit slices, 4 blood orange slices
Once the citrus bars are completely cooled and topped with the citrus slices, transfer to a cutting board and cut into squares or rectangles.