Onion and garlic both contain fructans(oligosaccharides) and therefore during a restrictive phase of the low FODMAP diet, they should be excluded from the diet.
Onion is one of the greatest contributors to IBS symptoms. Strictly avoiding onion means not only cooking without onion (all varieties, plus leeks, shallots, and the white part of spring onions) but also avoiding packaged foods that contain onion ingredients such as onion powder in soups and stocks.
The fructan content in onion and garlic are soluble in water. This means that if you put onion or garlic into a soup or stock, some of the fructan content will leach out into the water. Therefore, the strategy of putting a whole onion or garlic clove into a soup and then pulling the pieces out before consuming the soup will not work, as the fructan content will have already leached into the water. Again, if you cook with onion but leave it on your plate, this will only reduce the fructan load rather than remove it.
In an oil-based dish, the fructans will not leach out (as fructans are not soluble in oil). Therefore, if you are making something based in oil, for example, a stir-fry, it is possible to add a large piece of onion or a whole garlic clove and simply pull the pieces out before adding other ingredients. This way you will have the flavour without the fructan content leeching into the meal.
Here are a few more tricks of the trade to get flavour in your cooking.
Use the green parts of spring onion or chives (including garlic chives) – add later in the cooking process as they take less cooking compare to onion
Asafoetida powder which is an Indian spice has an oniony flavour. Use sparely as is quite potent. You can buy it from most big supermarkets or from an Indian supermarket.
Garlic infused olive oil can be used instead of your regular oil. You can buy it from most big supermarkets. Avoid making homemade garlic-infused oil as there is a risk of botulism.
Use other low FODMAP flavours include ginger, fresh herbs, spices, lemon and lime juice if you are missing the flavour of onion and garlic.
To find out more about the low FODMAP diet, book a FREE 15-minute telephone appointment with me https://www.cambridgedietitian.co.uk/booknow/