In a large bowl, mix together the eggs, sugar, oil, and orange blossom water. Add the flour, baking powder and salt and mix until the dough comes together. Cover the bowl with plastic wrap and place in the fridge for one hour to firm up.
Meanwhile, make the filling. Place the almonds, walnut pieces and pistachio nut into the bowl of a food processor fitted with a metal blade. Process until the pieces are no large than ¼-inch big. Add the sugar and egg whites and pulse briefly to combine and the mixture comes together.
Preheat oven to 350ºF. Line 2 to 3 large cookie sheets with parchment paper or silicone baking mats, or bake in batches. Divide the dough in half.
Take another two pieces of parchment and sprinkle flour on one, place one dough half on top, and then sprinkle a little more flour on top of the dough. Place the second piece of parchment on top of the dough and roll on top of the parchment to roll out the dough until it is about 1/4-inch thick. Every few rolls, peel back the top parchment and sprinkle a little more flour on the dough.
Use a 3-inch drinking glass or round cookie cutter to cut the dough into circles. Use a metal flat blade spatula to lift up the circle and place on another spot on the parchment that has been sprinkled with a little flour.
Place about one heaping teaspoon of the filling in the center and then fold in the three sides towards the middle to form a triangle, leaving a small opening in the center. Pinch the three sides together very tightly. Place on the prepared cookie sheets. Repeat with the remaining dough and roll and cut any dough scraps you have, making sure to sprinkle a little flour under and over the dough before you roll.
Bake for 16 minutes, or until lightly browned. While the cookies are baking, place the sugar, lemon juice and water into a small saucepan and bring to a boil. Stir once or twice. Once boiling, cook on medium heat for 4 minutes, or until mixture thickens. Remove from heat and add the orange blossom water. Set aside.
When the cookies are baked, slide the parchment paper onto wire racks. Use a pastry brush to brush the glaze over the top of the cookies. Let cool and enjoy.
Store in an airtight container at room temperature for up to five days or freeze for up to three months.