In a bowl, whisk the stock with the cornstarch. In a large skillet, heat the oil. Add the garlic and cook over high heat until fragrant, 20 seconds. Add the bok choy and stir-fry until the leaves start to wilt, 2 minutes. Add the stock and cook, stirring, until slightly thickened, 30 seconds. Remove the bok choy from the heat, season with salt and pepper and serve.