In the bowl of a stand mixer, whisk together the butter, milk, yeast, sugar, salt, egg, and yolk until blended. Add the flour, and use a dough hook to bring the mixture together and knead on low speed for five to seven minutes, until the dough is stretchy but soft. Transfer it to an oiled bowl, cover tightly with plastic, and set aside for one and a half to two hours in a warm spot, until just about doubled—it will grow from about two cups to about four. Additional advice: If your kitchen runs cold, or things seem to be moving very slowly, heat your oven to 200 degrees Fahrenheit (95 degrees Celsius) for a few minutes, turn it off, place your bowl in there, and let it stay there for the remainder of the rising time. It should move along more quickly.

(For a longer rise, or to finish baking the following day, you can chill the dough in the fridge. Just remember to take it out about one and a half hours before baking, so it has time to warm up again before you roll it out.)

While the dough rises, cook your onions: Melt the butter in a large sauté pan over medium heat. Add the onions, toss to coat them in the butter, and cover the pot. Reduce the heat to medium-low, and let the onions 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 the salt. Cook the onions, stirring every 5 minutes, for another 20 to 30 minutes, until they are golden brown and very tender and sweet. No need to fully caramelize them, as you would for onion soup, 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 five minutes. Transfer them to a plate, and spread them out so that they cool faster.

On a large, well-floured counter, roll out the dough until it is about 12 inches wide on the side closest to you and as far away from you as you can make it when rolling it very thin, likely 10 to 14 inches. Spoon and then spread the onions over the dough in an even layer; then sprinkle the onions with two teaspoons poppy seeds. Roll up the dough with the filling, rolling it away from you into a tight coil. Transfer the coil to a Gefen Parchment-lined baking sheet or board, place it in your freezer, and let it chill for five to 10 minutes. (You will be able to cut it in half much more cleanly if the dough has been chilled.) While it’s there . . .

Prepare the 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.

Remove the dough from the freezer, and use a serrated knife to cut the log gently lengthwise into two long strips; 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 to your prepared loaf pan.

Let the dough proof again: Cover the pan with the same plastic wrap you used earlier, 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 degrees Fahrenheit (175 degrees Celsius).

Sprinkle the babka with a few extra 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 until the internal temperature is 185 degrees Fahrenheit (85 degrees Celsius). If the onions get too brown on top (mine did), you can cover the babka with foil for the last few minutes, but unless they fully burn, it will taste great regardless.

Let it cool for 10 to 20 minutes, then cut it 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.