Preheat the oven to 450°F / 230°C.
In a large roasting pan or oven-safe casserole, combine the onion, garlic, eggplant, zucchini, potato, and tomatoes.
Drizzle with all but one tablespoon of the olive oil.
Season generously with a large pinch of salt, pepper, and the cumin.
Toss well to ensure all vegetables are coated.
Roast for about 45 minutes, shaking the pan or stirring occasionally, until the vegetables are nicely browned and caramelized at the edges.
Carefully transfer the roasting pan to the stovetop over medium heat.
Add the vegetable stock and the reserved tomato juices.
Bring to a simmer and cook, stirring occasionally, for another 15 minutes, or until the vegetables are very soft.
For a creamier texture, cool the soup slightly, then use an immersion blender to purée about half of it directly in the pot. If you prefer a chunkier, more rustic soup, you can skip this step entirely.
Stir in the chopped parsley and the remaining tablespoon of olive oil.
Taste and adjust seasoning with more salt and pepper if needed.
Serve hot.
Don't overcrowd the roasting pan. Use two if necessary to ensure the vegetables roast and caramelize rather than steam.
