Sauté the Onions: Begin by heating oil over medium heat in a 4 to 5 quart soup pot or Dutch oven until the surface shimmers. Add the diced onions to the pot and allow them to sweat, stirring frequently, for about 5 minutes or until they soften.
Incorporate Aromatics: Add the Thai red curry paste, chopped ginger, optional lemongrass, and finely chopped garlic into the pot. Stir these ingredients into the onions and cook for a minute or two until the mixture becomes fragrant and well combined.
Add Liquids and Simmer: Increase the heat to medium-high and pour in the chicken broth and coconut milk. Bring the soup to a low boil, then reduce the heat to maintain an active simmer. This process helps blend the flavors beautifully.
Add Dumplings and Season: Stir in the fish sauce and sugar, then add the frozen dumplings to the simmering soup. Allow the soup to return to a bubbling state and cook the dumplings according to their packaging instructions. Most dumplings just need a few minutes, as any meat they contain is usually pre-cooked (always double-check to be safe).
Add Fresh Ingredients: Turn off the heat and add the chopped spinach, sliced green onions, and chopped cilantro into the pot. Let the soup stand for a moment until the spinach wilts naturally. Squeeze the juice from half of a lime into the soup and stir well. Taste the soup and, if needed, add more lime juice for extra zest.
Serve and Garnish: Ladle the soup into bowls and top with your favorite toppings such as fresh herbs, chili slices, or crispy shallots. Serve hot and enjoy the rich, flavorful comfort of this Thai-inspired soup.