Place the rinsed red lentils, diced onion, minced garlic, grated ginger, and diced tomatoes with their juices into the slow cooker.
Add the vegetable broth, cumin, turmeric, garam masala, chili powder, salt, and black pepper. Stir once to combine.
Cover and cook on low for 6 to 7 hours until the lentils are soft and the stew has thickened.
Stir in the chopped spinach during the last 15 minutes of cooking so it wilts but stays bright green.
Taste and adjust salt or pepper if needed before serving.