Well, Westeros leans more to harvest festivals than “Thanksgiving,” but I would imagine neeps (and probably parsnips too, based on English Christmas meals I’ve had where the potato and yam are less represented than in the U.S) are pretty heavily represented as a starchy side-dish, since potatoes don’t exist in Westeros (nor do any New World crops).
However, we don’t have to guess, since we have an example of a Northern harvest feast from Bran III of ACOK:
Such food Bran had never seen; course after course after course, so much that he could not manage more than a bite or two of each dish. There were great joints of aurochs roasted with leeks, venison pies chunky with carrots, bacon, and mushrooms, mutton chops sauced in honey and cloves, savory duck, peppered boar, goose, skewers of pigeon and capon, beef-and-barley stew, cold fruit soup. Lord Wyman had brought twenty casks of fish from White Harbor packed in salt and seaweed; whitefish and winkles, crabs and mussels, clams, herring, cod, salmon, lobster and lampreys. There was black bread and honeycakes and oaten biscuits; there were turnips and pease and beets, beans and squash and huge red onions; there were baked apples and berry tarts and pears poached in strongwine. Wheels of white cheese were set at every table, above and below the salt, and flagons of hot spice wine and chilled autumn ale were passed up and down the tables.
…The serving men brought every dish to Bran first, that he might take the lord’s portion if he chose. By the time they reached the ducks, he could eat no more. After that he nodded approval at each course in turn, and waved it away. If the dish smelled especially choice, he would send it to one of the lords on the dais, a gesture of friendship and favor that Maester Luwin told him he must make. He sent some salmon down to poor sad Lady Hornwood, the boar to the boisterous Umbers, a dish of goose-in-berries to Cley Cerwyn, and a huge lobster to Joseth the master of horse, who was neither lord nor guest, but had seen to Dancer’s training and made it possible for Bran to ride. He sent sweets to Hodor and Old Nan as well, for no reason but he loved them. Ser Rodrik reminded him to send something to his foster brothers, so he sent Little Walder some boiled beets and Big Walder the buttered turnips.
So if we look at the menu, you’ll notice that the meats tend towards game animals – aurochs, venison, duck, fish, pigeon, boar – although there are some stock animals like beef and mutton and poultry as well, which points to the necessity of trying to keep as much of the livestock alive through the winter as possible so that you can continue to get eggs, milk, cheese, and wool off of them.
As far as starches go, in addition to the trenchers which serve as plate and bread at the same time, you have black bread and oaten biscuits, which speaks to the northern climate where hardier grains like oats and rye do better than more fragile wheat. There’s a lot of starchy veg as well, with leeks, turnips/neeps, (parsnips don’t show up here but they do in other feasts), etc. “Pease” refers to pease porridge, which is usually made with bacon or ham as well as carrots, onions, turnips, and various spices.
Also as you might expect from a harvest meal, there’s a lot of seasonal fruits, both used as sauces and served on their own, and it’s all richly spiced both for flavor and as a preservative b/c food cannot be wasted.