Heat some oil in a saucepan and sauté the sliced onion and minced ginger until softened. Meanwhile, in another pot, cook the noodles according to package instructions.
Add red curry paste, brown sugar, stock, soy sauce, and vinegar to the saucepan. Add sliced mushrooms, shrimp, and red bell peppers. Cook for about 3 minutes.
Prepare a slurry using cornstarch and water and slowly add it in, stirring continuously. Turn down the heat and cook on low until the soup thickens.
Add the drained noodles and coconut milk and stir. Season, taste and adjust. Take it off heat and garnish with fresh herbs and lime juice.
Notes
Cook the noodles separately to prevent them from becoming overly soft.
Add the coconut milk after removing the soup from the heat.
Adjust the red curry paste according to your preferred spice level.
Taste and adjust the seasoning before serving, as the saltiness of different stocks and soy sauces can vary.
For the best texture, add noodles to individual bowls if you expect leftovers.