Rinse the sprouts and pull off any loose brown husks floating on the surface — they turn bitter when cooked.
Put the sprouts, water, and 1/2 tsp salt in a pot and cover with a tight-fitting lid.
Bring to a boil over high heat and cook covered for exactly 5 minutes without lifting the lid.
Turn off the heat and let the pot sit covered for 1 more minute before you open it.
Drain in a colander, saving the cooking liquid for soup stock if you'd like, and spread the sprouts out to cool slightly.
While the sprouts are still faintly warm, add garlic, sesame oil, scallion, sesame seeds, and salt to taste.
Toss gently with your hands or two spoons until evenly coated, without crushing the sprouts.
