In a mixing bowl, combine the flour and salt. Gradually add water while mixing until a soft dough forms. Knead for 5 minutes, then cover and let it rest for 15 minutes.
Prepare your filling of choice by mixing the ingredients in a bowl. Set aside.
Roll the dough out on a lightly floured surface to about ⅛-inch thickness. Cut out circles using a cookie cutter or glass.
Place a small spoonful of filling in the center of each circle. Fold the dough over to create a half-moon shape and pinch the edges to seal. Set aside.
In a large pot, heat the vegetable oil over medium heat. Add the diced onion and sauté for 3-4 minutes until translucent.
Add the minced garlic and ginger, cooking for an additional minute until fragrant.
Stir in the red curry paste, cooking for 1-2 minutes to bloom the spices.
Pour in the chicken or vegetable broth and coconut milk. Bring to a gentle simmer.
Add the fish sauce (or soy sauce), lime juice, mushrooms, and bell pepper to the pot. Let simmer for 10 minutes until vegetables are tender.
Gently add the dumplings to the soup, cooking for an additional 8-10 minutes until they float to the surface and are cooked through.
Stir in the chopped spinach or bok choy and cook for another 2 minutes until wilted.
Ladle the soup into bowls and garnish with fresh cilantro. Serve hot.