Place the lamb in a large skillet over medium heat and brown on all sides for about 5 minutes. Transfer the meat to the slow cooker.
Add the diced onion, minced garlic, red curry paste, butternut squash, coconut milk, beef broth, salt, and black pepper to the slow cooker.
Stir everything together until the curry paste is evenly distributed.
Cover and cook on low for 8 hours or until the lamb is fork-tender and the squash is soft.
Taste and adjust salt if needed. Sprinkle with chopped cilantro just before serving.