Answer: Onions are high in carbs but still fit into keto meals in small portions.
Like many root vegetables, onions are high in carbs. One small onion contains 5.3 g of net carbs. Fortunately, few people are tempted to eat an entire onion in one sitting.
People typically eat small amounts of onion in their meals. Perhaps you want a few slices of onions to accompany your steak. Each medium onion slice is around 1 g of net carbs, so it's doable, but not free.
Onions are a tremendously popular ingredient in keto recipes. Onions are great for keto cooking because a small amount adds lots of flavor. You can often add a half or whole onion to a multi-serving recipe without bloating the number of carbs in each serving.