Cut 1 lb flank steak into two equal halves, against the grain. Each half should be 2-3 inches long. Cover with cold water (about 2 cups) and boil for 30 minutes (covered) until a fork slides through with some resistance. The meat should not be bouncy or hard. But it shouldn't be completely tender either.
Meanwhile, hard boil the quail eggs: cover with cold water and bring to a boil. Simmer for two minutes. Drain and immediately transfer to an ice water bath. Chill for 5 minutes. Peel eggs and set aside.
When cool enough to handle, cut or shred beef into bite-sized strips. (The traditional way is to shred the beef by hand but you can also cut it into neat strips. Cutting instead of hand shredding avoids stringy pieces of beef.)
Add braising liquid: soy sauce, mirin, and sugar. Simmer (uncovered) for another 15 minutes until the beef is dark and there is just a little bit of resistance when you slide in a fork. Half the original liquid should be evaporated by now.
Add shishito peppers and simmer (covered) until soft and deflated, another 10-15 minutes. By now, the beef should be tender with very little resistance. The liquid should be 20% of the original volume.
Add hard-boiled quail eggs and cook another 5-10 minutes, making sure to spoon the braising liquid over the eggs.
Serve warm, cold, or room temperature -- preferably with rice and other banchan. Enjoy!
Jangjorim should last about 1 week in the fridge, tightly covered.
Feel free to add more water to the soy sauce braise if it evaporates too quickly.