For dough for one loaf -- must rise before kneading:
Heat milk and butter gently in a saucepan until the butter has melted. In a large bowl, add the milk-butter mix to the sugar, salt, and water. When cooled below 115 degrees, add the yeast, two cups of the flour, and the egg. Beat about 100 strokes and start adding more flour to make a soft dough, shaggy dough. Cover and let rise in a warm place 1 hour, then knead 10 minutes, adding more flour if necessary to make a dough that is soft but not too sticky to work with.
While the dough is rising the first time, make the filling:
Melt the butter. Remove from heat and add the other ingredients.
To assemble the loaf:
Divide the dough in thirds. Roll out each third to a rectangle about 12 inches long and 5 or 6 inches wide. Spread with 1/3 of the filling, making sure to keep substantial margins free of filling. Roll the dough into fairly tight foot-long tube. Pinch the seam and tuck in the ends.
Lightly grease a cookie sheet (I'd recommend one with raised edges, in case the butter leaks out) and sprinkle with cornmeal. To braid the loaf, tuck the end of one tube into the end of another, forming an elongated V. Lay the third tube down the middle of the V, toward one side. Lift the outer tube nearest the middle tube over it and place it in the V, near the other outer tube. Then lift that outer tube over what's now the middle tube and place it in the V, near the other outer tube. And so on. When you get to the ends of the tubes, which will happen quickly, tuck them together.
Cover with a clean tea-towel and let rise 45 minutes or so. Preheat the oven to 375. When the loaf looks like it's risen nicely, beat the white of one egg with a tablespoon or so of cold water, brush on the dough, and sprikle with poppy seeds. Bake 30 to 40 minutes -- I'd check after 30 and see how it looks. You don't want it to burn but you don't want it uncooked in the middle either.
Variation: Fill with basil pesto instead of the onion mixture.
Cautionary note: If the butter leaks out onto the cookie sheet, it will burn and fill your kitchen with smoke. Be ready to open a window, or all the windows you have available.
Onion bread, Ontario Road, April 2010.