1. Assemble the challah: Turn the dough out of the bowl onto a clean, unfloured work surface. Using a bench scraper or sharp knife, divide it into 8 equal portions for two medium loaves, or 4 portions for one large loaf. If you’re making two medium loaves, set half the portions aside and keep them covered with a towel while you work. Using a rolling pin and working with one portion at a time, roll each into a 5-by-8-inch rectangle (8-by-12-inch rectangle for a large loaf).
2. Prepare the filling and continue assembling: In a small bowl, stir together the brown sugar and cinnamon. Leaving at least a 1-inch border around the edges, sprinkle the mixture evenly over each rectangle of dough, then scatter the raisins on top. 
3. Starting at a long edge, roll it into a tight log. Gently stretch and roll with your hands to shape it into a 10- to 12-inch strand (if making one large loaf, stretch to 22 to 24 inches). Repeat with the reserved dough portions.
4. Form one woven round: Lay 2 dough ropes vertically on your work surface, spaced about 1½ inches apart. Place 2 more ropes horizontally across them so they intersect in a grid. Arrange the ropes so they alternate going over and under one another, just like a woven pie crust. Working counterclockwise, fold each “under” rope over the “over” rope directly to its left. Repeat this fold at each intersection, working your way around the loaf in one full circle of weaving. Once you’ve completed the circle, a new set of ropes will now be the “unders.” Switch directions (now you’re going clockwise) and repeat the process, folding each new “under” rope over the “over” rope to its right. Continue this back-and-forth pattern (one full circle, then switch directions) until the ropes are too short to fold any farther, then tuck any loose ends underneath the finished round.
5. Repeat this braiding process with the remaining dough to make a second loaf. Transfer both to a parchment paper–­ lined baking sheet, cover loosely, and let rise in a warm spot for 45 minutes to 1 hour, until soft and puffy.
6. Preheat the oven to 350°F. In a small bowl, make the egg wash: Whisk together the egg with 1 tablespoon of water. Brush evenly over the exterior of each loaf. 
7. Bake until the outside is deeply golden and the loaf feels hollow when tapped (it should register an internal temperature of 190°F on an instant-­ read thermometer), 30 to 35 minutes for medium loaves and 35 to 40 minutes for a large loaf. Let the loaves cool for at least 30 minutes, then serve.

Notes: Once fully cooled, the challah can be wrapped well in plastic wrap and stored in an airtight container at room temperature for up to 3 days. For longer storage, wrap cooled loaves tightly in plastic wrap, followed by aluminum foil and a labeled freezer-­ safe bag with the air squeezed out; freeze for up to 2 months. To serve, thaw at room temperature (still wrapped) for at least 5 hours, then remove the bag, foil, and plastic wrap, transfer to a baking sheet, and refresh in a 350°F oven for 10 to 12 minutes. Avoid refrigerating the challah at any point; it’ll dry out.