In a mixer bowl, combine flour, sugar, and yeast and mix well, then add salt and mix until combined. Add egg, oil, and water. Fit the mixer with a dough hook and mix on low speed until the dough starts to come together, about 1-2 minutes. If the dough feels too dry, add a bit of water, and if too wet, add a bit of flour.
Keep kneading for about 8 minutes on medium-low speed, until the dough is soft and elastic and pulls away from the sides of the bowl. It's ok if it sticks to the bottom.
Place dough in a lightly greased bowl and cover with plastic wrap. Let rest in a warm place for 1-2 hours or until doubled in size.
Line a baking sheet with parchment paper. When the dough has risen, divide it into two or three even pieces, and shape each into braided challah (read the post above for more information on how it's done). Place challahs on prepared baking sheet, leaving as much space as possible between them, and cover loosely with plastic wrap. Let rise for about 60 minutes or until it has nearly doubled in size.
Preheat oven to 350°F/180°C.
Brush challah with an egg wash (1 egg mixed with 1 tablespoon water) for a shiny top, and sprinkle with seeds. Bake in the oven for 30 minutes or until golden brown. A skewer inserted into the center should come out clean. If you have an instant thermometer, the center of the loaf should register 200F/90C. Allow to cool completely on a wire rack before slicing.
Challa is best the same day it’s made, but can be frozen for up to 2 months.