Whisk together the yeast and 3/4 cup of bread flour. Sprinkle the yeast mixture over warm water in a small bowl (temperature no more than 100°F). Let stand for 5 minutes until creamy foam forms.

Combine the yeast mixture with eggs, vegetable oil, sugar, and salt in a mixing bowl. Add half remaining bread flour and stir. Gradually add the rest, mixing well. Knead on a floured surface until smooth for 8 minutes. Place in an oiled bowl, cover, and let rise in a warm place until doubled, about 2 hours.

Mix onion, poppy seeds, salt, all-purpose flour, and butter in a bowl. Divide onion mixture into 2 portions. Also, divide the dough into 2 equal strips (30x4 inch). Spoon filling lengthwise down the center of each strip. Wrap dough edges over filling, pinching to seal.

Arrange filled strands seam side down and braid them starting from the middle. Form into a ring and pinch edges together. Place the ring on a greased baking sheet, cover with plastic wrap, and allow it to triple in size in 1.5 hours.

Preheat the oven to 325°F (165°C). Arrange rack to the lowest position.

Beat an egg with salt and brush the top of the ring with this wash. Sprinkle additional poppy seeds if desired.