Rinse the zucchinis and mushrooms under cold water, then cut the zucchinis into half-moon shapes while slicing mushrooms and finely slicing the onion. Mince the garlic finely.
Heat a large skillet on medium flame and pour in the olive oil, letting it warm up for about a minute.
Toss in the sliced onion and cook, stirring occasionally, until it softens and turns translucent, around 3 minutes.
Add the garlic and mushrooms to the pan, continue cooking for 3 to 4 minutes until mushrooms soften and release their juices, stirring now and then.
Introduce the zucchini slices and season with salt, black pepper, and sprinkle thyme if using. Keep stirring frequently while cooking for another 3 to 5 minutes until the zucchini becomes tender yet maintains some firmness.
Taste and adjust the seasoning with a pinch more salt or pepper as preferred.
Remove the skillet from heat, transfer the vegetables onto a serving dish, sprinkle with fresh chopped parsley if desired, and serve immediately while warm.
