Make the dough: Combine flour, granulated sugar, zest, and yeast together in the bowl of a stand mixer. Add water and eggs and mix with dough hook on low speed until the dry and liquid ingredients come together. Mix in the salt. Then add the butter, a tablespoon at a time, incorporating each addition fully before adding the next tablespoon. Knead with the dough hook on low speed until the dough looks smooth and pulls away from the sides of the bowl. (For me, this was less than 5 minutes of kneading after all the butter was incorporated.) Place dough in large greased bowl, cover with plastic wrap, and place in the refrigerator for at least 10 hours or preferably overnight.

Make the filling: Place walnuts in food processor and pulse until chopped into small pieces. Remove and set aside about half the chopped walnuts. To the remaining walnuts in the food processor, add the brown sugar, butter, cinnamon, and tahini. Process until a paste is formed.

Preheat the oven and assemble the loaves: Grease two standard loaf pans and line bottoms with parchment paper. Remove half the dough from the refrigerator (leave the other half in, for now). On a lightly floured surface, roll the first portion of dough into a rectangle about 12 inches long and 10 inches wide. Spread half the walnut-brown-sugar-tahini-butter paste over the rectangle, leaving about a 1/2 inch border. Sprinkle on half the chopped walnuts and half the crumbled halva, pressing slightly to adhere. Brush the short edge of the rectangle that is farthest away from you with a bit of water. Now, starting with the short end nearest to you, roll the dough into a log. Place the log in the freezer for 15 minutes to firm up a bit. Repeat with the second portion of dough. Remove each log from the freezer, trim about 1/2 inch off each end, and slice the log in half lengthwise. Then, twist the two halves of the logs together, trying to keep the cut sides facing out. Place the twisted loaf in a loaf pan. (You can either nestle the bits trimmed off the end in the same loaf pan, or bake them separately as a little "mini babka.") Cover with damp towel and allow the loaves to rise at room temperature for 1 to 1.5 hours.

Bake the loaves: Preheat the oven to 375 degrees Fahrenheit. Bake the loaves for 40-45 minutes, or until a wooden skewer can be inserted with little resistance, and comes out free of dough. (You will probably need to cover the loaves with foil for the last 10 minutes of the baking time to prevent overbrowning.) After removing from the oven, immediately brush the loaves with sugar syrup (see below), then allow the loaves to cool for 20 minutes in the pans, on a wire rack, before removing them from the pans to cool completely.

To make the sugar syrup: While the loaves are baking, combine the 6 tablespoons sugar and 1/3 cup of water in a small saucepan, and heat over low heat until the sugar completely dissolves. Remove from the heat and set aside.