Preheat the oven to 400 degrees F. Spray or grease a large, rimmed baking sheet.
Chop the zucchini and/or summer squash into bite-size pieces. Cut the mushrooms in half (or quarters if large). Cut the onion in half, and then slice from root to tip end. Separate the slices. Place all the veggies in a large mixing bowl.
Sprinkle the Italian seasoning, salt and pepper over the vegetables, and then drizzle with the olive oil and balsamic vinegar. Toss to evenly coat (using your clean hands makes this easy). Transfer the mixture to the prepared baking sheet, and spread into an even layer. Roast for 20 minutes, and then stir and redistribute the vegetables. (There will likely be some liquid on the surface of the baking sheet at this point.) Continue to roast until the surface of the baking sheet looks dry and the zucchini and mushrooms are beginning to brown, 15-20 minutes more.**
You may sprinkle the feta over top after cooking, but I prefer to add it in the final few minutes of baking. Optionally, to add a little extra browning to the vegetables and feta, sprinkle the feta over all and then broil for a minute or two, watching very carefully to avoid burning, or until the vegetables are lightly golden in spots.
Remove from the oven, sprinkle with the chopped herbs, and enjoy. Leftovers improve with age and are delicious cold or gently warmed.