I would be a bit wary. Some restaurants maintain a stock of sauce which can have onions or garlic in it.

I am of Indian origin, but not Jain, and have never tried this - but I did work at an Indian restaurant in college.

I’d call ahead to just confirm to be safe.

I'd also be careful of counting on this. I'm intolerant of garlic and several Indian places have told me there weren't *any* menu items without it (other than plain naan). One fancy Indian place downtown marinades 100% of their chicken in a garlic marinade days in advance! Fortunately, I'd called a few days before my work was going, so they were able to set something aside for me. Can't remember the name - it was a few years ago.

Low FODMAP cookbooks and food brands are going to be your friend! Fody Foods is one of the brands I use. There's also some weird rule about how garlic in olive oil can be used to infuse it with the taste, but the harmful chemical gets filtered out? I'm only intolerant rather than allergic, so I'd do some more research on this one. Fody makes tons of marinades and sauces that should be safe for you and they are carried in some grocery stores (very hit or miss - there's a lot more on the website).