Caramelize the Onions
Heat 2 tbsp oil or butter in a skillet over medium-low.
Add sliced onions + a pinch of salt. Cook slowly, stirring often, until deeply golden and sweet, about 20–25 minutes.
Cool completely before adding to dough.
Make the Honey & Yeast Mixtures
In a small bowl, mix 1/3 cup honey, 1/3 cup oil, a pinch of salt, and 1/4 cup lukewarm water.
In another bowl, dissolve 2 tsp (2 packets) yeast in 1/4 cup lukewarm water. Let it sit until foamy, 5–10 minutes.
Mix the Dough
In the bowl of a stand mixer, Using the dough hook attachment combine the honey mixture + yeast mixture.
Beat in 1 egg + 2 yolks.
Add flour gradually, alternating with buttermilk, until you have a smooth, elastic dough (not sticky). Knead 3–5 minutes in mixer, or 7 minutes by hand.
Remove the dough from the bowl of the stand mixer.
Toss the onions in a bit of flour, and gently knead them into the dough using your hands, you many need more flour as the onions add moisture.
First Rise
Transfer dough to a greased bowl, cover with plastic wrap, and refrigerate at least 2 hours (overnight is even better for flavor).
Shape the Challah
Punch down the dough. On a lightly floured surface, divide into 3 equal strands (about 2.5 inches wide and 14 inches long).
Roll into three ropes
Pinch the tops of all three ropes together, then bread the ropes, and pinch the bottom ends together and tuck underneath.
Second Rise
Place braided loaf on a parchment-lined baking sheet. Cover and let rise 30–45 minutes, until puffy.
Preheat oven to 350°F (175°C).
Whisk egg white with a splash of water and brush over the top.
Sprinkle generously with sesame seeds.
Bake for 30 minutes, until golden (don’t let it get too dark).
Optional: Brush with melted butter after taking it out of the oven