Heat the olive oil in a large lidded shallow casserole or pot over medium heat. Add the onion and fry for a few minutes until softened.
Add the garlic and ginger and cook for about a minute until fragrant.
Add the Cape Malay curry powder and turmeric and cook briefly, stirring, until fragrant.
Add the diced tomatoes and sugar and stir to combine.
Pour in the coconut milk and season with salt. Allow the sauce to bubble gently for about 10 minutes.
Stir through the garam masala and check the seasoning, adjusting to taste.
Add the mussels and close the lid. Cook for about 8 to 10 minutes until all the shells have opened. Discard any mussels that remain closed.
Serve in deep bowls scattered with fresh coriander, with crusty bread or French fries alongside to mop up the sauce.