Preheat oven to 350°F (175°C). Line an 8x8 inch baking pan with parchment paper, leaving an overhang for easy removal.

Prepare the crust: In a medium bowl, combine flour, chilled butter, sugar, and salt. Use your fingers or a pastry blender to cut the butter into the dry ingredients until the mixture resembles coarse crumbs. Press firmly and evenly into the bottom of the prepared pan.

Bake the crust for 15-18 minutes, or until lightly golden brown around the edges. Remove from the oven and reduce the oven temperature to 325°F (160°C).

Prepare the filling: While the crust bakes, whisk together sugar, eggs, flour, lemon juice, and lemon zest in a bowl until completely smooth. Pour the lemon filling evenly over the hot, partially baked crust.

Prepare the nectarines: In a small skillet over medium heat, melt 2 tablespoons of butter. Add brown sugar and stir until dissolved. Add the nectarine slices and sauté for 2-3 minutes per side until slightly softened and caramelized. Remove from heat.

Top the lemon bars by arranging the caramelized nectarine slices decoratively over the liquid filling. Be careful not to disturb the structure too much.

Bake the bars at 325°F (160°C) for 30-35 minutes, or until the edges are set but the center still has a slight jiggle. Let cool completely on a wire rack, then chill in the refrigerator for at least 3 hours before cutting into bars.