To make the buns, preheat the oven to 425°F (220°C). Line a small baking sheet with parchment paper. Place the sweet potato on the prepared baking sheet. Bake the sweet potato for 20 minutes, until it is fork-tender.
Let it cool enough to handle, and then scrape out the flesh. Transfer the sweet potato flesh to a blender and blend until it is smooth. Measure ½ cup (122 g) of the sweet potato puree, then set the puree aside.
Next, whisk together the all-purpose flour, bread flour, and salt in a medium bowl. Set the flour mixture aside.
In a medium skillet over medium heat, melt 2 tablespoons (28 g) of the butter. Add the onion and sauté it for 20 minutes, or until it has become a caramel color.
Remove the skillet from the heat and allow the onion to cool to room temperature.
Prepare your dough by first stirring together the yeast, water, and honey in the bowl of a stand mixer fitted with the paddle attachment. Let the yeast mixture sit for about 10 minutes, until it is foamy.
While the yeast mixture is resting, lightly grease a large bowl with oil. Set the bowl aside.
Add the remaining 6 tablespoons (84 g) of butter, eggs, sweet potato puree, and sautéed onion. Mix the ingredients together at medium speed until they are combined.
Fit the stand mixer with the dough hook attachment and slowly spoon in the flour mixture while mixing the ingredients at medium-low speed.
Once all of the flour is added, increase the mixer’s speed to medium-high and beat the dough for 6 to 8 minutes, until a smooth, elastic dough forms and pulls away from the sides of the bowl. Transfer the dough to the prepared bowl.
Place the bowl in a warm, draft-free location to allow the dough to rise for 1 to 2 hours, or until it has doubled in size.
Line a rimmed large baking sheet with parchment paper. Lightly dust a work surface with flour.
Then lightly coat the top of your hand with flour. Gently press down on the dough with your floured fist to release any excess air.
Weigh the dough and divide it into 12 equal pieces. Cup the palm of your hand over a piece of dough and lightly roll it in a circular motion on the prepared work surface to create a smooth ball.
Repeat this process with the remaining dough.
Place the buns on the prepared baking sheet. Loosely cover the buns with a light tea towel and allow them to rise in a warm, draft-free area for 30 minutes, or until they have doubled in size.
Meanwhile, make the topping. In a small bowl, stir together the salt, poppy seeds, chia seeds, sesame seeds, and onion powder. Set the topping aside.
Preheat the oven to 350°F (180°C). While the oven is preheating, make the egg wash. In a small bowl, use a fork to stir together the egg and water. Very gently brush each bun with the egg wash, and then sprinkle the buns with the topping.
Bake the buns on the oven’s middle rack for 28 to 30 minutes, or until they are golden brown.
Transfer the baking sheet to a wire rack to cool for 5 minutes, then transfer the buns to the rack to completely cool.