Heat the olive oil in a large pot over medium heat. Add the onion and garlic and cook until softened, about 5 minutes.
Stir in the red curry paste and cook for 1 minute.
Pour in the vegetable broth, coconut milk, diced tomatoes with juice, fish sauce, brown sugar and lime juice. Bring to a boil then reduce heat to low and simmer for 20 minutes.
Stir in chili flakes and season with salt to taste.