Red Lentil and Sweet Potato Curry

• 1 brown onion
• 4 garlic cloves
• 1 tsp cumin seeds
• 1 tsp black mustard seeds
• 10 curry leaves (optional)
• 1/2 tsp chilli flakes (optional)
• 1 tsp turmeric
• 1/2 tsp garam masala
• 1 tbsp curry powder
• 1 tbsp tomato paste
• 1 cup red lentils
• 2 – 3 cups water
• 1 can coconut milk
• 1 large sweet potato
• 1 tsp sea salt (more to taste)
• Black pepper to taste
• Fresh lime