Place a heavy skillet on the stove top and adjust to medium-high heat. When skillet is hot, add sesame seeds and toast until they are golden brown, about 3 to 5 minutes. Then remove them from the pan and set them aside.
Next, add 1 tablespoon of water and the onions to the hot skillet. Cook until the onions are caramelized (browned around the edges and soft), then remove them from the pan and set them aside.
Next, add 1 tablespoon water to the hot skillet and place the Brussels sprouts face down, cooking until caramelized, about 7 to 9 minutes. Adding a splash of water at the very end will help the caramelizing process.
Remove the Brussels sprouts from the heat. Place in a large bowl and combine with the onions and toasted sesame seeds.