Family Tested & Approved
large egg yolk, reserve the white for the filling
unbleached all-purpose flour
rye flour (white, medium, or pumpernickel flour) or 1/2 cup whole wheat flour
large onions, peeled and chopped, about 4 cups
large egg white, reserved from dough
Combine all of the dough ingredients, and mix until the dough starts to leave the sides of the bowl. (If you're using a stand mixer, use the flat beater for this step.)
Cover the bowl and let this shaggy mass rest for 15 minutes or so, to absorb the liquid and make it easier to knead.
Knead the dough until it's smooth and slightly sticky, about 7 minutes in a stand mixer equipped with a dough hook.
Place the dough in a lightly greased bowl or 8-cup measure, cover the container with plastic wrap, and let the dough rise for 1 hour, or until it's increased in volume by at least a third.
While the dough is rising, make the filling. Fry the bacon (or bake it in a 350°F oven for about 35 minutes), until it's crisp and light brown.
Fry the onions until they're a rich, golden brown in some of the bacon fat, if desired; or fry them in olive oil. They should brown nicely in about 20 minutes.
Crumble the bacon, and combine it with the onions. When barely lukewarm, stir in the reserved egg white; this will help keep the filling from spilling out of the rolls.
Gently deflate the risen dough, and roll it into a 12" x 18" rectangle.
Spread the filling over the dough, and roll it up the long way, to make a log that's a generous 18" long.
Cut the log into 1" slices. Lay the 18 slices into two 9" round lightly greased cake pans.
Cover the pans, and let the rolls rise for about an hour, until they're nicely puffy. Toward the end of the rising time, preheat the oven to 375°F.
Uncover the rolls, and bake them for about 25 minutes, until they're golden brown.
Remove them from the oven, and brush them with butter, if desired. Serve warm or at room temperature.
Store leftovers in the refrigerator; to reheat, tent with aluminum foil, and bake in a preheated 350°F oven for about 10 minutes.