In a heavy casserole or Dutch oven lay at the bottom the carrots and the onions, season with salt and pepper and put on top the shanks. Add the garlic, 200 ml of the beer and the lemon peel. Insert a thermometer probe to one of the shanks, not touching the bone. Close tightly with the lid and cook for 40 minutes.
Reduce heat to 150 degrees C, and cook aiming for 80 degrees C, about 3 hours.
Halfway through open the lid and add the remaining beer.
When done, take the shanks on a plate and discard the lemon peel. Place in a medium saucepan the juices, the carrots and the onions. On high heat boil for 20 minutes until reduced to half. Return all in ingredients to a fine sieve and put your sauce in a sauce serving bowl.
In the meantime, baste the shanks with the remaining baste and return them the empty casserole, Increase the oven heat to 200 degrees C, and while uncovered brown the shanks for about 15 minutes.