In the bowl of a stand mixer, add starter, water, and sugar together and mix. Add flour, salt, sesame seeds, poppy seeds, onion flakes, and garlic powder. Knead for a few minutes; it's a really stiff dough. Cover and let rise 8-10 hours in a warm place or overnight if it's cooler in your house.
Dump dough onto counter and divide into 12-18 balls, depending on how big of a bagel you like. Roll into a log about 6 inches long, and then kind of loop the rope around your hand so the ends are overlapped under your fingers, and roll them together on the counter to create a bagel. This is the fastest way to make smooth bagels, but you can also do this; roll each hunk of dough into a ball. Poke a finger through the middle of the ball and spin it around to create a bigger loop. Once you master the log roll method, you will much prefer it!
Let bagels rest 2-3 hours, covered, on the counter.
When ready to boil, bring 2-3 quarts of water and ¼ cup of sugar to a boil in a 1 gallon or bigger pot. Gently drop bagels in, 5-6 at a time, boiling 1 minute on either side. When they're done boiling, put on a parchment or silpat lined tray.
When all the bagels are done, brush with egg wash, sprinkle with optional extra seeds. Seeds are beautiful and texturally fun but make a huge mess anytime someone cuts, toasts and butters a bagel…so if it's kids eating them, I lean towards no seeds to keep a semblance of countertop sanity.
Bake at 400F for 20-25 minutes, turning pans around halfway. They should be just starting to brown.