Coat the bottom of a large pot with oil and place over medium heat. Add onion and sauté until soft, about 5 minutes. Add garlic, ginger, berbere spice and cayenne and sauté 1 minute more. Add broth, lentils, tomatoes and potatoes. Stir a few times, raise heat and bring to a simmer. Lower heat and allow to cook until lentils are very soft and potatoes are tender, about 30 minutes. Stir in spinach and continue to cook just until wilted, about 2 minutes. Remove from heat and season with salt and pepper to taste. You can also add a bit more berbere and cayenne, if desired. Serve.