Rub lamb shanks with a bit of olive oil and season with salt and pepper
Pour the remaining olive oil in a Dutch oven and heat over medium-high heat
Add the lamb shanks and brown on all sides, about 4 minutes per side
In the meantime, prepare the marinade by combining balsamic vinegar, red wine, chicken broth, garlic and rosemary leaves; mix until thoroughly incorporated
Remove lamb shanks from the pot and set aside
Add the onions to the hot oil; cook for 2 minutes, frequently stirring, until onions begin to soften
Pour in the prepared marinade and return lamb shanks to the pot; bring to a boil and cook for 2 minutes
Cover with a lid and place in the oven; cook for 1.5 hours
Remove the lid, flip the lamb shanks to the other side, and continue to cook for 30 more minutes
Remove from oven and serve over mashed potatoes drizzled with the cooking liquid
Consumer Advisory: Consuming raw or undercooked meats, poultry, seafood, shellfish, or eggs may increase your risk of food borne illness, especially if you have certain medical conditions.