Butter a 9 by 12 1/2-inch baking pan. Line pan with foil and leave about 2 inches extra on all sides. Butter the foil.
Whisk the flour, cornstarch, and salt together in a bowl.
In a standing mixer fitted with the paddle attachment or with a hand mixer, beat the butter with the sugar on medium, until light and fluffy, about 4 minutes. Turn mixer to low speed, add the dry ingredients and mix until just combined. Do not over work the dough.
Transfer dough to prepared pan, spreading it out evenly across the pan and pressing to make a 1/4-inch thick covering over the bottom of the pan. Cover with plastic wrap, then press and flatten using your hands over the top to smooth and even out the dough. Refrigerate for 1 hour.
Preheat oven to 325 degrees F.
Remove the plastic from the top of the dough and prick the dough with a fork or a wooden skewer. Bake the crust until just golden, about 30 minutes.
Meanwhile, melt the jelly in the microwave until soft enough to spread, about 1 minute. Spread the jelly with a pastry brush or an offset spatula over the crust. Return to the oven and cook another 10 minutes. Cool crust completely on a wire rack, 1 hour.
Meanwhile, whisk the eggs, yolks, sugar, sweetened condensed milk, flour, and salt together in a medium bowl. Beat until thick and slightly pale. Stir in the citrus juices and zest, and whisk gently to blend well.
Pour the filling onto the cooled crust and bake until the filling is firm but still loose and wobbly in the center, about 25 to 30 minutes. Cool to room temperature, about 30 minutes. Chill in the refrigerator for at least 1 hour or overnight before cutting.
Remove pan from refrigerator and dip the bottom of it in warm water to soften the butter and release the foil from the sides of the pan. Use foil to lift pastry out of the baking dish and transfer to a cutting board. Using a sharp knife or a pizza cutter, cut into about 50 (1 1/2-inch) squares, wiping down the knife with a hot, wet towel between cuts. Top each square with a tiny triangular segment of clementine or blood orange (with the peel on) and serve.