Heat the olive oil in a large skillet over medium-high heat. Add the beef cubes and cook until browned on all sides, about 5 minutes. Transfer the beef to the slow cooker.
Add the onion, garlic, carrots, red bell pepper, lentils, berbere spice, diced tomatoes, and beef broth to the slow cooker. Stir to combine.
Cover and cook on low for 7 to 8 hours or on high for 4 hours, until the beef is tender and the lentils are soft.
Stir in the kale during the last 15 minutes of cooking so it wilts but stays bright green.
Taste and season with salt and black pepper as needed before serving.