Stir in the turmeric, cumin, cinnamon, and cardamom until combined. Continue cooking for about 1 minute, until fragrant.
Add the diced tomatoes (with juices), entire can of coconut milk, red lentils, broth, salt, and plenty of pepper. Add red pepper flakes or cayenne, if desired, to taste. Stir to combine. Increase heat to high and bring to a low boil.
Once it boils, reduce the heat to medium-high, and simmer, uncovered, for about 18 to 22 minutes, until the lentils are fluffy and tender.
Turn off the heat and stir in the spinach until wilted. Add the lime juice to taste. Taste and add more salt and pepper, if desired. Ladle into bowls and serve with toasted bread and lime wedges.