Place a large pot over medium-high heat and add the oil. When it is hot add the onions, garlic, curry leaves, garam masala, mustard seeds, paprika and turmeric and cook for about 3 minutes, stirring occasionally. Add the tomatoes and tomato paste and season with salt and pepper. Stir and simmer for 5 minutes, and then add the sweet potatoes, vegetable broth and yogurt. Cover and continue to cook until the sweet potatoes are tender, 20 to 25 minutes. Add the mussels and replace the lid. Cook until the mussels have opened, 5 to 8 minutes. Serve the mussels with lots of the curry on rice, roti or toasted bread.