Brown the lamb chunks in a Dutch oven over medium-high heat until seared on all sides. Remove the lamb and set it aside.
Add the diced onion to the pot and cook until softened. Stir in the garlic and ginger, then cook for one minute more.
Mix in the red curry paste and let it cook for 30 seconds to release its aroma. Pour in the coconut milk and beef broth, scraping the bottom of the pot to loosen any browned bits.
Return the lamb to the pot. Bring the mixture to a simmer, then cover and cook on low heat for 60 minutes until the lamb starts to tenderize.
Add the sliced red bell pepper and carrots. Continue simmering uncovered for 20 more minutes until the vegetables soften and the gravy thickens.
Stir in the fish sauce, brown sugar, and lime juice. Taste and adjust seasoning if needed. Serve the stew over cooked jasmine rice and top with fresh cilantro and Thai basil.