Prepare your meat by trimming the excess fat off and cutting the beef into 1 ½ inch cubes. Season the meat with salt and pepper.
Heat vegetable oil in a large heavy bottom pan over medium-high heat. Sear the beef chunks until well golden brown (about 4 minutes on both sides is enough). Don’t overcrowd the pan, you’ll need to do this in batches, carefully cleaning the surface of the pan with a paper towel if necessary and adding more oil before the next batch. Don’t change the heat temperature and don’t overcook the beef. Transfer to slow 4.5-quart cooker.
Add chopped potatoes, carrots.
In the same pan (cleaned slightly with a paper towel), saute onion until softened and add red curry paste and garlic. Saute 2 minutes more on medium-low. Transfer to slow cooker.
Add spices, broth, lime juice, coconut milk, sugar, soy sauce, cinnamon stick to the slow cooker. If using kaffir leaves, add them now.
Cook on LOW for 8 hours or HIGH 4 hours. After that taste for seasoning and add more salt/pepper if necessary. Top with green beans and cook on HIGH for another 15 minutes or LOW for 30 minutes. Taste for seasoning again.
Garnish Massaman Beef Curry with plenty of fresh herbs such as cilantro, thai basil, mint, parsley, lime wedges, more crushed red chili flakes, peanuts. Serve as it is with bread, grains, rice.