Heat oil in a large skillet over medium heat. Stir in onions and garlic, and cook until tender, about 5 minutes. Stir in mushrooms, 1/2 cup water, wine, and chicken bouillon. Season with basil, salt, and pepper. Bring to a boil; reduce heat, and simmer, uncovered, 10 minutes, stirring occasionally. Mix together cornstarch with 1/4 cup water. Stir into mushrooms, and cook until thickened, about 5 minutes.