[For a longer rise, or to use this tomorrow, you can chill the dough in the fridge. If you are, take it out about 1 1/2 hours before using it so it has time to warm up again before rolling it out.]
While dough rises, cook your onions: Melt butter in a large sauté pan over medium heat. Add the onions, toss to coat them in butter and cover the pot. Reduce the heat to medium-low and let them slowly steep for 12 to 15 minutes, stirring once or twice. You can walk away.
Uncover the pot, raise the heat slightly and stir in salt. Cook onions, stirring every 5 minutes for another 20 to 30 minutes, until golden brown and very tender and sweet. No need to fully caramelize them, as you would for onion soup or an irate French culinary instructor, which would take much longer. Raise the heat to medium-high and cook the onions until they get a little dark at the edges, about another 5 minutes. Transfer onions to a plate and spread them out so that they cool faster.
Assemble babka: On a large, well-floured counter, roll out dough until it is about 12 inches wide (the side closest to you) and as long in length (away from you) as you can when rolling it thin, likely 10 to 14 inches. Spoon then spread onions over dough in an even layer, then sprinkle onions with 2 teaspoons poppy seeds. Roll the dough up with the filling away from you into a tight coil. Transfer coil to a parchment-lined baking sheet or board to your freezer, just for 5 to 10 minutes. [It will cut much more cleanly in half when chilled.] While it’s there…
Prepare pan: Coat a standard loaf pan with butter or nonstick spray, and line the bottom and two sides with a sling of parchment paper for easier removal.
Finish shaping babka: Remove dough from freezer and use a serrated knife to gently cut the log lengthwise into two long strips and lay them next to each other, cut sides up. Lift one side over the next, forming a twist and trying to keep the cut sides facing up (because they’re pretty). Don’t worry if this step seems messy; it will be gorgeous regardless. Transfer the twist into your prepared loaf pan.
Let proof again: Cover the pan with the same plastic wrap and let it rise another 45 minutes at room temperature. You won’t see much of a change in the size and that’s fine; we’re just letting the dough relax a little.
Heat your oven: To 350°F.
Bake babka(s): Sprinkle babka with an extra couple pinches of poppy seeds. Bake for 40 to 45 minutes, until a skewer inserted into the center doesn’t feel like it’s hitting sticky/rubbery dough, or the internal temperature is 185°F. If onions get too brown on top (mine did), you can put some foil over for the last few minutes, but unless they full burn, they won’t taste bad.
Serve: Let cool as long as you can stand in in the pan, then cut into thick slices with a serrated knife. Leftovers keep at room temperature for a few days; I usually wrap it in foil. Gently toast slices to rewarm.