Heat the oil in a medium-sized pot or deep skillet over medium heat.

Stir in the chopped onion and cook until it turns translucent, around 5 minutes. Add the minced garlic and grated ginger, cooking for another minute until they become fragrant.

Sprinkle in the curry powder, ground cumin, ground turmeric, ground cinnamon, ground allspice, and cayenne pepper. Stir well to coat the onions and release the spices' flavors.

Add the cubed sweet potato to the pot, stirring to combine with the spiced onion mixture. Pour in the diced tomatoes (along with their juice) and the coconut milk. Season with stevia, salt and black pepper to taste.

Bring the mixture to a boil, then reduce the heat to low. Cover the pot and let it simmer for about 15-20 minutes or until the sweet potato pieces are tender.

Gently stir in the sliced plantains. Cover and cook for another 5-10 minutes or until the plantains are soft but still hold their shape.

Taste the curry and adjust the seasoning if needed, adding more salt, pepper, or cayenne pepper to your liking. Serve the sweet potato and plantain curry over your choice of side, or enjoy it as is. Garnish with fresh chopped cilantro for freshness.