Warm the oil in a medium pot over medium heat.
Sauté the onion until translucent, about 3 to 4 minutes.
Stir in the garlic and ginger. Cook until fragrant, about 30 seconds.
Bloom the red curry paste in the pot, stirring for 1 minute.
Pour in broth and coconut milk. Whisk to dissolve the paste.
Season with fish sauce, soy sauce, and brown sugar. Bring to a gentle simmer.
Add mushrooms and bell pepper. Simmer until just tender, about 5 minutes.
Slip in the frozen dumplings. Cover and simmer until cooked, 6 to 8 minutes.
Stir in spinach to wilt. Add lime juice, salt, and pepper. Adjust to taste.
Ladle into bowls. Top with cilantro and green onions. Serve hot.