Stir-fried onions with mushrooms are not poisonous. Nutritionally speaking, there is no such thing as food incompatibility, so sautéing onions with shiitake mushrooms will not be poisonous. Shiitake mushrooms are a kind of common edible fungi, each 100g of mushrooms only contains 32.7 calories, which is a low-calorie food, including 91.7g of water, 5.2g of carbohydrates, 2.2g of proteins and 1.0mg of vitamins. While every 100g of onion contains about 42.8 calories, of which 89% is water, in addition, it also contains 9.0g of carbohydrates, as well as a variety of vitamins and minerals. Therefore, mushrooms fried onions have a certain nutritional value, and after people consume them, they can replenish energy and water for the organism and provide the organism with the nutrients needed for metabolism. However, it should be noted that mushrooms fried onions should be eaten in moderation, and those who are allergic to both should abstain from eating.