Dice and sauté the onion in the olive oil and 1 tsp. salt. Divide the sautéed onion into three equal portions. Leave ⅓ in the pan and set the rest aside. Peel and dice the mushrooms and add to the pan with the onion. Sauté until soft and add salt to taste. Set aside. Peel and boil the potatoes. Drain and mash while still hot. Add in half of the sautéed onion (with the oil it was fried in) and salt to taste. Set aside. Boil the broccoli until tender. Puree until smooth. Salt to taste and mix in the remaining sautéed onion. Crack and gently beat the eggs. Pour a little into the potatoes and mix through. This will make the potato mixture smooth and creamy. Pour a small amount of egg mixture into the broccoli, and also into the mushrooms. (The broccoli and mushrooms will need much less than the potato—you don’t want them to be too liquidy.)Set the rest of the beaten egg aside for the egg wash later on. Lay out the puff pastry squares on a piece of parchment paper. Allow to defrost until pliable. Place a spoonful of mixture in the center of each square. Gently fold over in a triangle shape and press down to seal edges. If the dough feels dry, dip your fingertip in cold water, and that will help seal the edges. Transfer the bourekas to a baking sheet lined with parchment paper, and brush with egg. Optional: sprinkle seeds on top. I like to use sesame seeds for the potato and poppy seeds for the mushroom, and I leave the broccoli ones plain. Bake at 400° F for 30 minutes until tops are golden brown. Serve warm or at room temperature. For best results, reheat in the oven.