A lot depends on what area you're talking about. Are you in Texas or California?
If you can find an El Pollo Loco restaurant, you won't be disappointed. I think they're only in California so far.............
Their marinade makes for the tastiest chicken anywhere IMHO. I think they use Spanish saffron, which is $200 an ounce.
But for Fridays, they also have a number of shrimp offerings that are very good, too.
And their salsa bar has some of the best Pico de Gallo you'll find. That might be a little spicy for you though.
The avocado-green salsa (looks pale/white green) is not spicy at all, but very delicious.
And don't forget to ask for some CREAMY CILANTRO at the register. You have to ASK for it.
(Creamy cilantro salad dressing is very popular with people who don't like spicy or onion or garlic!)
If it's some other Mexican place you go to, if you like shrimp, I recommend getting the Shrimp Cocktail, which is not alcoholic.
Some restaurants don't offer the "coktel camarones" like El Pollo Loco doesn't.
And it might not be on the menu, but if you ask for it, they can do it anyway. (Not at EPL - must not have the ingredients)
It's a tall, heavy glass (usually) filled with something like V-8 Juice, cooked shrimp, cilantro, and a few veggies, but it's amazing.
It's usually served with saltine crackers, which are entirely optional. You might be able to get corn chips instead of the saltines.
Otherwise, any fish (or shrimp) cooked "al mojo de ajo" (sautee'd in garlic butter) will be sure to please.
Just noticed you steer clear of garlic/onion.
So the roast chicken is probably your best bet. I know onion-opposed people who really like the roast chicken.
Also, pico de gallo salsa has diced onion in it.
Actually, the guacamole salsa at EPL also has diced onion.
You might like some of the salsas, but you'll have to ask for which ones are NOT SPICY. They have a few (very few!)
The Mexican restaurants in the USA are very accustomed to having people ask for no spicy food. It's a challenge but they can do it.