Modern food tends to conceal the raw onion, but it's still there. If you buy any pre-packed salad (at least in the British Isles, and in every part of Europe I've been in), it will almost certainly contain a few segments of raw onion. The less strong tasting spring onion is often served as a garnish on soups and all kinds of other dishes.

Consumption of raw onions by simply peeling an onion and eating it like an apple continues in many parts of the world, particularly where people still grow their own. I've seen allotment gardeners do so here in Ireland, my grandfather did so, and indeed I do so myself, although I prefer small ones that I can put in my mouth whole, rather than biting into bigger ones. Generally, onions eaten this way are of a milder sort. There is some evidence to support the idea that onions were eaten this way in various historical eras, but it's hard to go from the presence of onion skin remnants to a conclusion that they were eaten raw - although there are plenty of ways in which you demonstrate that they were cooked.

Onions are one of the few vegetables which reasonably reliably grow in every part of Europe, and were found wild in some form in many parts. They're a fundamental taste in a great many cuisines because of that, and there are plenty of recipes for them. The chances are that Martin is looking to represent people being really hungry, or people so ignorant of cuisine they don't know that onions are ordinary cooked. In Sam Tarly's case, I'm guessing the former.

Sources:

Vera K Niñez, Household Gardens: Theoretical Considerations on an Old Survival Strategy

Susan Lyons, ‘Food plants, fruits and foreign foodstuffs: the archaeological evidence from urban medieval Ireland’ in Proceedings of the Royal Irish Academy. Section C: Archaeology, Celtic Studies, History, Linguistics, Literature, Vol. 115C, Food and Drink in Ireland (2015), pp. 111-166

C. M. Woolgar, D. Serjeantson, and T. Waldron, Food in Medieval England: Diet and Nutrition