For the dough: In a small bowl place the yeast, ½ teaspoon sugar, and lukewarm water. Stir gently to mix. Allow to sit 5 to 10 minutes until it becomes foamy on top.
In a large bowl or stand mixer fitted with whisk attachment, mix together 1 ½ cups of the flour, ½ cup sugar, onion powder, and salt. Add the water-yeast mixture, oil, and garlic to the flour. Mix thoroughly.
Add another 1 ½ cups of the flour and 2 eggs and mix until smooth. Switch to the dough hook attachment if you are using a stand mixer. Add the remaining flour 1 cup at a time and let the mixer run until the dough is smooth and sticks just slightly. If not using a mixer, use a wooden spoon until the dough starts to come together, and then dump out onto a floured work surface. Knead the dough using the heel of your hand until dough is lump-free. Depending on humidity, time of year, and the flour you use, you may need slightly more or less flour. You can add 1 tablespoon at a time until desired consistency.
Place dough in a greased bowl and cover with a towel. Allow to rise 3 hours.
Remove dough from the bowl, punch down, and cut into 10 to 12 equal pieces, 3 to 3.5 ounces each (use a food scale for an exact measurement). Roll each section in a rope and shape into a knotted roll. See pages 40–41 for shaping. Place on a baking sheet lined with parchment paper or silicone baking mat. Allow challah rolls to rise another 20 to 30 minutes. Preheat oven to 375°F while the dough rises.
For the topping: Beat 2 egg yolks with 1 teaspoon water in a small bowl. Using a pastry brush, glaze the challah with two coatings of egg wash. Sprinkle with dried onion flakes and sea salt.
Bake for 25 minutes or until just golden on top and challah feels light and hollow. Cool on a wire baking rack.