Okay, here’s the deal. Oyster mushrooms are tasty and meaty and also provide bulk for all the missing sautéed onions. They also have been lab tested by Monash University and have a low FODMAP serving size of 1 cup, chopped, or 75 g. That is a lot! The issue is that they might be hard to find, and they are not inexpensive.   Here is my suggestion if you're taking an economical approach. Monash University has also lab tested conventional canned mushrooms and if you drain them, you can also use up to 75 g of those per low FODMAP serving. The caveat is that I do not think they have the best texture and they do not have much flavor, so I would not use them for the entire amount of oyster mushrooms called for. Use some oyster mushrooms and make up the difference with the drained canned mushrooms, if you would like to be more economical.