Whisk together the yeast and 100g of bread flour. Sprinkle the yeast mixture over warm water in a small bowl. The water should be no more than 40 degrees C. Let stand for 5 minutes until the yeast softens and begins to form a creamy foam.
Combine the yeast mixture with 2 eggs, vegetable oil, sugar and 1 1/2 teaspoon of salt in a large mixing bowl; stir well to combine. Stir in half of the remaining bread flour until no dry spots remain. Stir in the remaining bread flour, a 70g at a time, mixing well after each addition. When the dough has pulled together, turn it out onto a lightly floured surface and knead until smooth and elastic, about 8 minutes. Lightly oil a large bowl, then place the dough in the bowl and turn to coat with oil. Cover with a light cloth and let rise in a warm place (27 to 35 degrees C) until doubled in volume, about 2 hours.
Line and generously grease a baking tray. Set aside. Mix onion, poppy seeds, 1/4 teaspoon salt, plain flour and butter together in a bowl. Divide onion mixture into 2 equal portions. Set aside. Divide dough into 2 equal portions. Roll each portion into approximately 60x10cm strips. Spoon the onion mixture down the centre of each strip lengthways. Begin pulling the edges of the dough up to wrap around the filling, pinching the edges to close securely.
Arrange the strands side by side, seam side down. Beginning in the middle of each strand, plait the strands together by crisscrossing each side, making sure to keep the seam side down. Once both sides are plaited, pinch the edges together to form a ring. Place the ring on the prepared baking tray and cover with cling. Allow ring to rise until tripled in volume, about 1 1/2 hours.
Preheat an oven to 170 C / Gas 3. Arrange the oven rack to the lowest position. Beat 1 egg and 1 pinch of salt together in a bowl. Brush the plaited ring with the egg mixture. Sprinkle with additional poppy seeds, if desired.
Bake in preheated oven until the top is golden brown and the bottom of the loaf sounds hollow when tapped, turning the pan around half way through so the ring browns evenly, 45 to 50 minutes.
1 (7g) sachet quick yeast is equal to 2 1/2 teaspoons.