Start by peeling and dicing the medium sweet potato and yellow onion. Mince the garlic and ginger, setting them aside for later.
In a large pot, heat a drizzle of oil over medium heat. Add the diced onion and sauté until it becomes translucent, about 5 minutes. Then, stir in the minced garlic and ginger, cooking for an additional minute until fragrant.
Sprinkle in the ground turmeric, cayenne pepper, black mustard seeds, and ground cumin. Stir well to coat the onions and garlic in the spices. Add the split red lentils, stirring to combine everything evenly.
Pour in the crushed tomatoes, coconut milk, and water. Stir to combine, then bring the mixture to a gentle boil.
Once boiling, reduce the heat to low and let the soup simmer for about 25-30 minutes. This allows the lentils to cook through and the flavors to meld beautifully.
After the lentils are tender, stir in the diced sweet potato and baby spinach. Cook for an additional 10-15 minutes, until the sweet potatoes are soft and the spinach has wilted.
Remove the pot from the heat and stir in the fresh lime juice and sea salt to taste. If desired, add ground black pepper and adjust other seasonings according to your preference.
Ladle the soup into bowls and garnish with chopped cilantro. Serve warm, and enjoy the comforting flavors of your homemade Lentil Soup with Coconut, Spinach & Lime!