Vegetables with the same texture can be roasted together. This means that root vegetables such as beets, carrots and potatoes can be roasted together and tender ones like tomatoes, zucchini and mushrooms can be roasted together, but these two categories of veggies should (ideally) not be combined on the same baking sheet. However, for this recipe, I had a combination and wanted to use only one baking sheet so I used baby potatoes which cook quicker and sliced the red onion into larger pieces so they won't brown quickly.