In a large pot, heat the coconut oil over medium heat. Add the diced onion and sauté until golden brown, about 5 minutes.
Stir in the minced garlic and grated ginger, cooking for an additional 1-2 minutes until fragrant.
Add the curry powder, ground turmeric, and cumin seeds to the pot. Cook while stirring for about 1 minute to release the flavors.
Add the rinsed lentils and diced tomatoes (with their juices) to the pot. Stir everything together to combine well.
Add the coconut milk and vegetable broth. Bring the mixture to a gentle boil, then reduce heat to low and let it simmer uncovered for about 20-25 minutes, or until the lentils are tender and the curry has thickened. Stir occasionally to prevent sticking.
Once the lentils are cooked, stir in the chopped spinach and cook for an additional 2-3 minutes until the spinach is wilted.
Remove the pot from heat and add lime juice. Season with salt and pepper to taste.
Serve the curry in bowls, topped with fresh cilantro for added color and flavor.