In a flameproof casserole cook the onions and the garlic in 2 tablespoons of the butter over moderately low heat, stirring, until the onions are softened, transfer the vegetables to a bowl, and reserve them. Heat the remaining 2 tablespoons butter in the casserole over moderately high heat until the foam subsides and in it brown the chuck, patted dry and seasoned with salt and pepper. Add the reserved vegetables, the thyme, the sage, the bay leaf, the broth, and the Guinness stout, bring the liquid to a boil and simmer the mixture, covered, stirring occasionally, for 1 to 1 1/2 hours, or until the meat is tender. Discard the bay leaf, bring the liquid to a boil and whisk in bits of the beurre manie, whisking until the sauce is thickened. Divide the stew and the potatoes between 2 heated bowls and sprinkle them with the parsley.