Pat the lamb shanks dry and generously season them with salt and pepper.
Heat olive oil in your heavy pan over medium-high heat until shimmering, then add the shanks. Sear until deeply golden brown on all sides, about 4-5 minutes per side, creating a flavorful crust.
Remove the shanks and set aside. In the same pan, add chopped onions and cook until they soften and become fragrant, about 8 minutes. They should turn a light caramel color and smell sweet.
Pour in the red wine, scraping up any browned bits from the bottom of the pan to build flavor. Bring to a gentle simmer and cook until the wine reduces by half, about 10 minutes, until it thickens and becomes syrupy.
Add the tomato paste, broth, and the crushed sprigs of rosemary and thyme. Stir well to combine, then nestle the lamb shanks back into the pan, ensuring they’re partially submerged in the sauce.
Cover the pan tightly with a lid or foil and transfer it to your preheated oven at 160°C (320°F). Braise for 2.5 to 3 hours, or until the meat is tender and falling off the bone, checking occasionally and turning the shanks gently during cooking.
Once cooked, carefully remove the shanks and set them on a serving plate. Tent loosely with foil and let rest for about 10 minutes. Meanwhile, place the pan back on the stove over medium heat to reduce the sauce if needed, until glossy and thickened.
Spoon the rich, wine-infused sauce over the lamb shanks and serve hot, ideally alongside mashed potatoes or crusty bread to soak up every bit of the flavorful gravy.