Start by washing the zucchinis and mushrooms thoroughly. Slice the zucchinis into half-moons, slice the mushrooms, and thinly slice the onion. Mince the garlic.
Place a large skillet over medium heat. Add the olive oil and allow it to heat for about 1 minute.
Add the sliced onions to the skillet. Sauté them for about 3 minutes until they start to soften and become translucent.
Add the minced garlic and sliced mushrooms to the skillet. Cook for an additional 3–4 minutes, stirring occasionally until the mushrooms are tender.
Add the sliced zucchinis into the mix. Season with salt, black pepper, and thyme if using. Sauté for another 3–5 minutes while stirring frequently.
Check your seasoning before removing from heat—adjust with more salt or pepper if needed. Transfer your sautéed veggies to a serving dish and garnish with fresh parsley if desired.