Cut the onion into thin strips and the carrots into slices, and soften the onion in olive oil in a large pot until translucent.
Add the lamb shanks and brown them on all sides.
Grate the tomato, chop the garlic and stir them into the pot. Pour in the brandy, let the alcohol evaporate for a few moments, then add enough water to cover the meat.
Cook the shanks covered over low heat until tender, about 1 to 1.5 hours.
Toast the flour in a dry pan and stir it into the almost finished stew to thicken the sauce slightly.
For the mashed potatoes: boil the peeled potatoes in salted water until soft, drain, then mash smooth with the butter and milk.
Serve the shanks with the mashed potatoes, generously drizzled with the sauce.