Wash all the vegetables thoroughly, then slice them into bite-sized pieces. Heat oil in a pan and sauté chopped garlic and onion until fragrant. Add the mushrooms, then the squash, and toss everything together. Stir-fry over high heat to keep the vegetables crisp and vibrant.

When the squash softens, add the bell peppers and stir well. Next, add 2 tablespoons oyster sauce, 1 tablespoon sesame oil, 2 teaspoons bouillon powder, and a pinch of sugar. Adjust seasoning to taste. Finally, add thinly sliced onion and sprinkle black pepper on top before serving.