Heat vegetable oil in a large pot over medium heat. Add diced onion and sauté for 3-4 minutes until translucent.
Stir in minced garlic and grated ginger, cooking for an additional minute until fragrant.
Pour in chicken or vegetable broth and water, then add soy sauce and sesame oil. Bring to a gentle boil.
Carefully add frozen potstickers to the pot. Reduce heat to a simmer and cook for 8-10 minutes.
Add sliced mushrooms and baby spinach, cooking for another 2-3 minutes until spinach wilts.
Season with salt and pepper to taste, then serve garnished with sliced green onions and sesame seeds.