This Saturday-night stew is best started in the afternoon, as it takes 3 hours to slow-cook to perfection.
For the stew, heat the oven to 325º. Sprinkle the beef cubes with salt and set them aside. In a large frying pan, cook the onions in the olive oil over low to moderate heat, stirring occasionally and turning up the heat a bit to slightly caramelize them. Transfer the onions to a heavy, medium-size ovenproof pot with a lid.
In the frying pan, brown the salted beef (do not cook it through), in batches if you need to, over fairly high heat. Then transfer it to the pot with the onions.
Lower the heat under the frying pan, then add the brown sugar and stir it around. Add the flour and cook for 5 minutes, stirring constantly. Slowly pour in the beer (it will foam up), stirring and scraping to loosen caramelized bits from the pan. Stir in the stock and bring it to a boil, then remove it from the heat (don't worry if it's lumpy).
Taste the mixture and add more salt if desired. Add the bay leaves, thyme, and sage to the pot with the beef and onions and pour in the liquid from the frying pan. Bring the stew to a boil on the stovetop. Then transfer it to the oven, tightly covered, for about 3 hours.