Onion and garlic are both items that should be excluded while undergoing a low FODMAP diet. They contain fructo-oligosaccharides, which are difficult to digest in the small intestines, causing fermentation of gut bacteria, and worsening of IBS symptoms. Onion and garlic are commonly used seasonings, but can also be found hidden in other foods; such as salsas, pasta sauce, dips, etc. Some alternatives of spices to use while cooking may be chives, cumin, chili powder, lemon pepper, allspice, parsley, oregano, Italian seasoning, curry, amongst others and remember to always check the back of labels for the ingredients!
