Heat the vegetable oil in a large skillet or wok over medium-high heat.
Add the sliced onion and sauté for 2-3 minutes until softened.
Stir in the minced garlic and grated ginger, cooking for an additional 1 minute until fragrant.
Add the sliced cabbage, carrots, bell pepper, and snap peas to the skillet. Stir-fry for about 5-7 minutes, or until the vegetables are tender-crisp.
Pour in the soy sauce, rice vinegar, and sesame oil. Toss everything together and cook for another 2 minutes.
Season with salt and pepper to taste. If desired, sprinkle with sesame seeds.
Remove from heat and garnish with chopped green onions before serving.