Combine water, yeast, sugar and semolina flour in the work bowl of a stand mixer fitted with the dough hook; turn on briefly to combine, then let stand 15 minutes until bubbly.
Add bread flour and salt to bowl. Turn on to low and let run until dough comes together and starts to become stretchy. Add oil and turn up to medium or medium-high; knead until satiny, shiny, and very stretchy, 8-10 minutes.
Coat a bowl with a little olive oil. Form dough into a ball, and place in oiled bowl, turning to coat ball on all sides. Cover with plastic wrap and let stand at room temperature until doubled in size, about 1 hour.
Preheat oven to 325°F Scatter cornmeal over a baking sheet and set aside.
On a lightly floured surface, knead the dough briefly, then divide in 3 equal pieces. Roll each piece out between your hands and the board until it forms a rope about 15" in length. Place the three ropes side by side. Beginning from the center, braid the ropes, tucking the ends under when you reach them. Turn the loaf around and braid the other side. Try not to stretch the ropes too much, or it won't fit on your pan. (Braiding from the middle to the ends gives you a more uniform shape than braiding from end to end.) Place the loaf on the prepared baking sheet; you will probably have to place it diagonally. Squish or stretch as needed to make it fit; leave at least 2" at either end to allow the loaf to grow as it proofs. Cover loosely with plastic and let rise until doubled in size, about 45 minutes.
Sesame seeds are optional but traditional; if you wish to use them, beat egg and water and brush the bread generously, then sprinkle with sesame seeds. You can egg wash the bread without the sesame seeds, too, which will give it a shiny brown crust, but this is optional.
Bake in preheated oven 45 minutes or until golden brown and an instant read thermometer inserted into the center of the loaf reads 200°F.
Remove from oven, transfer to a rack and cool completely before slicing. (Hot bread is delicious, but it will squish and collapse if you cut into it before the structure has a chance to cool and set.).