Heat the oil on high heat in large skillet. Add the onions and zucchini. Sauté for 1 minute while shaking the pan to mix the vegetables. Mix in the diced tomato and garlic then cook for 1 minute more. Pour in the white wine and soy sauce then reduce for 1 minute while cooking on high. Toss in the cilantro. Thicken the mixture with the cornstarch slurry by pouring it into the skillet slowly while mixing. Add the sugar, butter, and black pepper then toss until the butter melts. Makes about 6 side dish servings.