Preheat the oven to 200°C (392°F).
Heat a dry frying pan on the highest setting.
Meanwhile, peel the potatoes, cut them in half, and slice them thinly.
Add 2 tbsp of olive oil to the hot pan and sauté the potatoes over medium heat until lightly browned. Season with salt and pepper.
Halve the zucchini lengthwise, peel the carrots, and slice both thinly.
Transfer the sautéed potatoes from the pan onto a baking sheet large enough to comfortably fit the fish.
Heat another 2 tbsp of olive oil in the same pan and sauté the zucchini and carrots until lightly browned. Season with salt and pepper, then add them to the potatoes on the baking sheet.
Peel the onions, cut them in half, and slice into strips. Mince the garlic. Add both to the vegetables on the tray, sprinkle the thyme over everything, and toss well to combine.