6 lamb shanks
Kosher salt
Freshly ground black pepper
2 tbsp. extra-virgin olive oil
4 tbsp. butter, divided
1 onion, chopped
2 medium carrots, peeled and finely chopped
3 cloves garlic, minced
2 tbsp. tomato paste
2 c. low-sodium beef broth
1 (15-oz.) can tomato sauce
2 sprigs rosemary
1 c. red wine
Mashed potatoes, for serving