In a small bowl, stir together the soy sauce, rice vinegar, brown sugar, toasted sesame oil, and optional crushed red pepper flakes until the sugar is mostly dissolved. Set aside.

Heat a 12-inch (30 cm) skillet or wok over medium-high heat. Add the pork sausage and cook for 6 to 8 minutes, breaking it into bite-size pieces as it browns. The sausage should be well browned with no raw pink areas remaining and must reach an internal temperature of 160°F (71°C). Transfer it to a plate with a slotted spoon.

Leave about 1 tablespoon of rendered fat in the skillet. If the sausage was very lean and the pan looks dry, add the 1 tablespoon (15 ml) neutral oil. If there is substantially more than 1 tablespoon of fat, carefully spoon out the excess.

Add the sliced onion and red bell pepper: Cook over medium-high heat for 3 to 4 minutes, stirring frequently, until the onion begins to soften and the pepper is slightly tender but still brightly colored.

Stir in the garlic and ginger: Cook for about 30 seconds, just until fragrant. Keep the mixture moving so the garlic does not brown or burn.

Add the cabbage in two or three additions if necessary, tossing after each addition as it begins to wilt. Cook for 5 to 7 minutes, stirring often. The cabbage should shrink noticeably and become tender around the edges while retaining a slight crunch. If it starts browning too quickly before it softens, reduce the heat to medium.

Return the browned sausage and any accumulated juices to the skillet. Pour the prepared sauce around the edges of the pan and toss everything together. Cook for another 1 to 2 minutes, until the sauce lightly coats the cabbage and the sausage is hot.

Remove the skillet from the heat: Stir in half of the sliced green onions and scatter the remaining green onions over the top. Serve immediately while the cabbage is still tender-crisp.