Celery, kohlrabi, broccoli, napa, bok choy, cabbage, summer squash & zucchini stir fry up nicely but you have to flash fry the squash (crank up the heat - I cook these last) or they tend to get too limp and soggy.
If you're not watching carbs, black beans & corn are fantastic in Mexican food. I live in New Mexico and they use it quite a bit here, even in salsa. I make my own black beans and cook them up with 2 pieces of chopped turkey bacon (browned before cooking) so it's low fat. Also, there's a popular root veggie called Jimica that ppl seem to like here. It's too carby for me, but they even make coleslaw with it here.
Have you tried using powdered onion? Maybe after it's processed it won't bother your tummy as much...just a thought.