caslon, i do mine in a pot on the stovetop, so i'm not sure how it translates to a casserole but here's how i make pot roast.
be sure to heavily salt and pepper the roast before browning. don't be shy. then make sure you get a good, dark browning on all sides of the meat.
when that's done, toast the garlic over low heat in oil and butter in a ss frying pan, and put into pot with the roast, then sweat the onions in the same pan. you can do all of this in one pot but it can get tricky and stuff can get burnt after a few things are cooked in the same vessel, so i like to use a separate pan. when enough fond is built up, i deglaze it with the red wine and add that to the pot.
in goes the carrots and celery, and add the stock and wine so that it just comes up to around 1/3rd the height of the meat. no more than 1/2 way up or you'll be boiling instead of braising when the meat shrinks and the veggies release their liquids.
strip the thyme leaves from about 3 or 4 of the stems and add to the pot. and mince no more than 1" of the rosemary spikes.
rosemary is strong, so go easy with it.
cover and let it simmer for an hour and a half, then turn the meat over and add the potatoes. if you put the spuds in from the beginning they'll break up and disintegrate into the liquid.
continue simmering until the meat is fork tender and the potatoes are cooked through.
when done, remove the meat and veggies, and crank the heat up to get the remaining liquid to boil and reduce by at least 1/3rd. taste and adjust with salt and pepper, then add a cornstarch slurry to thicken it into a gravy. you can also add butter for a richer gravy.
some people like to blend up the carrots, onions, and celery and strain to add to the liquid before reducing for gravy. i usually just serve them on the side.
also, instead of cooking my potatoes in with the roast, i maked mashed separately. nothing goes better with pot roast and veg than gravy smothered mashed potatoes.
hth.