Preheat oven to 375f
Place sweet potatoes in a large pot, cover with salted water and place on high for 15-20 minutes til potatoes soften.
preheat a large skillet on high, melt 1 tbsp butter then add lamb, carrots, garlic , thyme, rosemary, onions, salt, pepper, Worcestershire sauce. Cook, stirring til meat breaks apart and begins to brown, 8-10 minutes. Then add vegetables and broth, stir in flour, cover and simmer another 5-8 minutes.
remove lid and continue to simmer til vegetables soften and liquid reduces by half.
Drain sweet potatoes into a collander. Return pot to heat, reduce heat to medium-high. Melt remaining butter in pot, add kale and season with salt. Sautee, stirring occasionally til kale is fully wilted. 3-5 minutes.
Turn off heat to pot. Return potatoes to pot, add sour cream and mash roughly. Taste and adjust flavour with salt and pepper as desired.
Add lamb to a 13x9 baking dish, cover evenly with potatoes and bake at 375F for 20 minutes. Then optionally broil for another 3-5 minutes