Put the onion in a large flameproof casserole or deep skillet with a lid and cover it; place over medium heat and cook, checking and stirring every few minutes, until the onion is dry and almost sticking to the pan, about 15 minutes. At that point, add 2 TBS of the oil and cook, stirring occasionally, until the onion becomes a brown, soft mass, another 15 mins or so. Remove with a slotted spoon and add the remaining oil to the skillet.
Raise the heat to medium-high and brown the meat on all sides, sprinkling it with salt and pepper as it browns. When the meat is brown, add the garlic and cook, stirring, for about 30 seconds. Return the onion to the pan, along with the herbs and beer. Bring to a boil, then adjust the heat so the mixture simmers; cover.
Cook, checking occasionally to make sure the mixture does not dry out (unlikely, but if it does, add more beer), for about an hour, or until the meat is quite tender. Uncover the pan; the mixture should be stewy but not soupy; if it is, raise the heat and boil out some of the liquid. Stir in the mustard, then taste and adjust the seasoning. Garnish and serve or cool, cover, and refrigerate.