Cut the meat into large cubes, and put into a pot with along with 1 1/2 quarts water, the onion, bay leaf, as well as the other vegetables. Simmer, covered, until done. Remove the meat. Strain the broth and set aside. In a saucepan, melt the butter, add the flour and make a [light] roux. Add a bit of the broth [and stir until smooth]. In order to thicken the gravy, briefly bring it to a boil, and then stir in the egg yolk or saffron. Adjust seasonings with salt, pepper, a bit of vinegar, sugar, and lemon juice. Let the meat steep in the gravy a few minutes. Serve with 'Kartoffellaible' or pasta. Serves 4.