Heat the olive oil in a large skillet over medium heat.
Add the sliced onions and sauté for about 3-4 minutes until they start to soften.
Stir in the minced garlic and cook for an additional 30 seconds until fragrant.
Add the sliced mushrooms to the skillet and cook for 5-6 minutes until they release their moisture and begin to brown.
Add the sliced zucchini, soy sauce, black pepper, salt, and red pepper flakes (if using). Stir well to combine.
Cook for another 5-7 minutes, stirring occasionally, until the zucchini is tender but still slightly crisp.
Remove from heat and garnish with chopped green onions before serving.