In the bowl of a stand mixer fitted with the dough hook, mix the flour and yeast. Add the sugar, egg, and oil.

Start kneading and add the water gradually.

After 1 minute, add the salt. Keep kneading for about 10 minutes, until the dough is soft and elastic.

Lightly oil the dough, place it in a covered bowl, and let rise for 1–1½ hours, until doubled in size.

Divide the dough into 2 balls. Roll each one into a square about ½ inch (1.5 cm) thick.

Cut into long, narrow triangles. Roll each triangle up from the wide base into a small croissant.

Arrange the croissants on a parchment-lined baking sheet, snug against each other, so they join into one challah in the oven. Cover and let rise for about 40 minutes.

Brush with the egg yolk mixture and sprinkle with sesame or poppy seeds.

Bake in an oven preheated to 340–350°F (170–180°C) for 20–25 minutes, until the challah is golden and shiny.

As soon as it comes out of the oven, cover it with a clean kitchen towel to keep it soft.