Heat oil in a heavy-bottomed pot over medium-low heat. Add chopped onions, white parts of scallions, and garlic. Sauté until onions soften and release their natural sweetness.
Add sliced mushrooms and a pinch of salt. Cook for a few minutes until mushrooms soften and the flavors meld beautifully.
Stir in red Thai curry paste, sugar, and soy sauce. Mix well for a minute, then add vegetable broth and bring to a gentle simmer.
Pour in the coconut milk and stir to combine. Simmer on medium-low heat until the broth is creamy and aromatic.
Add frozen dumplings to the simmering broth. Let them cook for about 7 minutes, until fully thawed and tender.
Ladle the soup into bowls and top with scallion greens, chili oil, cilantro, and crispy garlic for a restaurant-style finish.