Place the diced onion, minced garlic, rinsed lentils, berbere spice blend, crushed tomatoes, vegetable broth, and salt directly into the slow cooker.
Stir everything together until the spices and tomatoes are evenly distributed through the liquid.
Cover and cook on low for 7 hours or on high for 3 1/2 to 4 hours, until the lentils are soft and the stew has thickened.
Taste and add a pinch more salt if needed.
Ladle into bowls and top each serving with a sprinkle of chopped cilantro.