Cut the meat into large cubes, and put into a pot with along with 1.5 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.