Heat oil in a large pot over medium heat.
When the oil is hot, add onion, garlic, and ginger. Cook and stir until the onions are tender and translucent.
Add carrots and bell pepper and cook for 3-4 minutes.
Next, add the curry powder followed by red curry paste. Cook for a minute or two or until aromatic.
Next, tip in the mushrooms and give a good stir to coat the mushrooms.
Add soy sauce.
Pour the broth, and add brown sugar (if using) and stir to combine. Taste the soup to make sure all the spices and salt are according to your taste. Add more, if required. Bring to a boil.
Add the coconut milk. Reduce the heat to low. Simmer for 5 minutes.
Serve hot garnished with That basil and cilantro. Serve lime/lemon wedges, if desired.