Yields 1 huge challah, enough slices for 40-50 people
To shape the challah, combine both doughs. Cut off one section of dough that weighs four and a half pounds (two kilograms), and a second section of dough that weighs six and a half pounds (three kilograms). (You’ll have about five pounds/two and a half kilograms of dough left over. Make it into some challahs for Shabbos.)
Divide the smallest section of dough into six even pieces. This will be the bottom of your challah, the part that makes it rise tall. Roll out each piece of dough and then roll it up like a snail.
Place the pieces in a pattern in the pan, one at each end and four in the center.
Take the second, larger piece of dough and divide it evenly into another six pieces. Roll each ball out and braid it into a large six-braid shape.
Gently pick up your six-braided challah and place it on top of the six pieces already in your pan.
Cover the pan with a large plastic bag and let it rise for about an hour to an hour and a half. It should rise to just a bit over the top of the pan, but not more than that — or else it will hit the top of your oven when it bakes.
(For a small European oven, you’ll need to remove every rack and leave only one rack in the very lowest place in the oven to fit it in.) Preheat the oven to 400°F (200°C).
Brush the challah very generously with two beaten eggs and then sprinkle on your seeds of choice. Slide the challah into the oven and set a timer for 20 minutes.
When the timer beeps, the top of your challah should be hard and turning brown. Cut a large piece of foil and tent it over the top of the challah. It’s not necessary to crimp it all over; just place it on top of the challah and shut the oven door.
Turn the heat down to 375°F (190°C). Set the timer for 1 hour and 20 minutes (total baking time for a challah this size and this depth is 1 hour 40 minutes). The challah will smell like it’s burning during the last stages of the baking process, but ignore this and let it finish baking completely.
When the timer beeps this time, remove the challah from the oven. (Use good oven mitts, as it will be very, very hot.) Prepare a large cooling rack on your countertop. Carefully flip the pan to its side and shake it to get the challah to slide out. Peel off the parchment paper, and flip the challah right side up. Leave it to cool completely on the rack for another hour or so.
To prepare this huge simchah challah, you’ll need the largest size challah pan, 20 inches (50 cm) long. Smear the pan with margarine so the Gefen Parchment Paper will stick really well. (This margarine will not touch your challah.) Line the entire pan well.
To freeze, place the challah in a very heavy-duty garbage bag and knot it on top. This challah needs four hours to defrost, so take it out with ample time before your simchah.