In a large pot, heat the vegetable oil over medium heat. Add the diced onion and sauté for about 3-4 minutes until softened.
Stir in the minced garlic and grated ginger, cooking for an additional 1-2 minutes until fragrant.
Pour in the chicken or vegetable broth and water, then add the soy sauce and sesame oil. Bring the mixture to a gentle boil.
Carefully add the frozen potstickers to the pot. Reduce the heat to a simmer and cook for about 8-10 minutes, or until the potstickers are heated through.
Stir in the baby spinach and cook for another 1-2 minutes until wilted. Season with salt and pepper to taste.
Ladle the soup into bowls, garnish with sliced green onions and sesame seeds if desired, and serve hot.