Rinse the zucchini and mushrooms well. Cut the zucchini into half-moon slices, slice the mushrooms, thinly cut the onion, and mince the garlic.
Set a large skillet over medium heat and pour in the olive oil. Let it warm up for about 1 minute.
Add the onion and cook for around 3 minutes, stirring now and then, until it softens and starts to turn translucent.
Stir in the garlic and mushrooms. Cook for 3 to 4 minutes, stirring occasionally, until the mushrooms soften and release their liquid.
Add the zucchini to the pan. Season with salt, black pepper, and thyme if using. Cook for another 3 to 5 minutes, stirring often, until the zucchini turns tender but still has a little bite.
Taste and adjust the seasoning as needed.
Remove from the heat, spoon into a serving dish, and sprinkle with fresh parsley if desired. Serve warm.