Wow - how do you like your onions, Id ask...
For example, if you're doing a stir-fry sort of recipe, I'd say you could add them almost together.
If you want a rich, caramelised sauce, I'd sweat the onions first, for about 15 minutes, then add the garlic, then add the chicken and some liquid..
Or perhaps I'd zap the onions & garlic together in the blender, quick fry for a few minutes then add the chicken...
Or I might just salt the chicken, cook it in my favourite oil/butter, set it aside then cook the onions & garlic in the same oil...
Is your meat in big pieces, or small pieces, or what?
So many combinations,so many winners!