Sweet & Savory Grapefruit Bars

For the topping: Scoop preserves into a small saucepan, and place on medium low heat till melted. Stir in grapefruit juice, zest and salt. Bring to a boil, and cook till it reduces and becomes slightly syrupy and thicker, 1-2 minutes. Let cool to room temperature while you're making the pastry.
For the pastry: Adjust oven rack to middle position. Heat oven to 350u0b0F. Butter or pan spray an 8- by 8- by 2-inch baking pan and line it with a piece of parchment long enough to overhang the pan's edges.
Combine flour, salt, grapefruit zest, feta cheese, and butter in the bowl of a food processor. Pulse to form a dry but pebbly mixture, about twelve 1-second pulses. You can also use a pastry blender or two knives to cut the butter and cheese into the dry ingredients. Pour into the prepared baking pan, and press into an even layer.
Spread liquid jam evenly over the prepared pastry. Sprinkle with chopped walnuts. Bake for 20-25 minutes. The edges will brown and come away from the sides a bit.
Remove the pan from the oven and place on a rack to cool. While it is warm, press any loose nuts into the pastry, sprinkle with flaky salt (if using), and run a knife along the edge of the pan so that it's easy to remove when cool. Let it cool completely before cutting.
To Serve: Carefully tug loose and lift using overhanging parchment and transfer to a cutting board. Cut into 16 two-inch squares (or any other shape you wish).