Preheat the oven to 425°F (220°C).
Chop the veggies into relatively even-sized pieces and toss them with olive oil (or a neutral oil like sunflower oil), a good pinch of salt, and black pepper. Arrange them on a roasting tray into rows.
Bell Peppers: First, slice the peppers, remove the seeds, and cut them into medium-sized pieces (as shown below). Transfer them to a bowl, add a tablespoon of light olive oil or a neutral oil (like sunflower oil), a
pinch of salt and pepper. Toss them and transfer them to the roasting
tray.
Eggplant: Cut the eggplant in half and then into half-inch slices. Toss them with a tablespoon of oil and a pinch of salt and pepper, and then transfer them to the roasting tray.
Zucchini: Cut the zucchini in half and then into half-inch slices. Toss them with a tablespoon of oil and a pinch of salt and pepper, and then transfer them to the roasting tray.
Red Onion: And finally, peel the red onion and slice it in half and then into fourths. Toss them with half a tablespoon of oil and a pinch of salt
and pepper, and then transfer them to the roasting tray.
The Mushrooms: Toss the mushrooms whole (if they're small) with half a tablespoon of oil and a pinch of salt and pepper, and then transfer them to a roasting tray.
Roast the vegetables in the oven at 425°F (220°C) until tender on the inside and a little charred on the outside. For the mushrooms, that’s about 25 minutes, although that will depend on the size of the mushrooms you’re using. And for the rest of the vegetables, it’s about 35 minutes.
While the veggies are roasting, make the vinaigrette dressing by whisking together the lemon juice and zest, mustard, oregano, minced garlic, extra-virgin olive oil, finely chopped fresh parsley, and a good pinch of salt and pepper.
Once you’ve pulled the roasted vegetables out of the oven, transfer them to a bowl and toss them with half the vinaigrette dressing. As they’re warm, they’ll absorb all that flavor. Let the veggies cool for about 5 minutes.
If you want to serve the salad warm, now, toss the vegetables with the spinach and the remaining vinaigrette. If you want to serve it cold, combine them just before serving. I prefer this roasted vegetable salad served warm but it’s also delicious when cold!