Can you make stew without onions?
If you were to cook meat in just water (possibly after browning it) you could call it a braise. In order to be a stew it probably needs some other ingredients. If you don't like or don't have onions, there's no need to use them. Use whatever vegetables you feel like using. They aren't serving a chemical purpose, such as leavening or thickening or browning; they're just for taste and to make a filling meal by adding things that cost less than meat. 

You can of course add a variety of spices, which may make your stew qualify as a curry or a chili. And if you intend to serve it over rice or noodles, you may want to thicken the liquid a little. There are many ways to do that, but white flour, predissolved in a little cold water, is a popular one. This is, for some people, a key discriminant between being a soup and being a stew: that stews are thickened with flour and soups are not. But you can find plenty of people to disagree with that.

Why is it important to you that you make something called a stew?