Equipment and preparation: You will need a large, solid baking sheet and a large, wide, flameproof casserole.
You can make the croûtons well ahead of time and, to do this, preheat the oven to 180C/350F/Gas 4. Drizzle the olive oil on to the baking sheet add the crushed garlic, then using either your hands or a piece of kitchen paper, spread the oil and garlic all over the baking sheet. Now place the bread slices on top of the oil, then turn them over so that both sides have been lightly coated with the oil. Bake for 20-25 minutes till crisp and crunchy.
When you're ready to cook the beef, lower the oven temperature to 150C/300F/Gas 2. Take the flameproof casserole, place it over direct heat, then heat the oil until sizzling hot and fry the meat, 3 or 4 pieces at a time, until they turn a dark mahogany colour on all sides. Make sure you don't overcrowd the pan or they will create steam and never become brown. As you brown the meat remove it to a plate then, when all the meat is ready, add the onions to the pan, still keeping the heat high. Toss them around until they become darkly tinged at the edges - this will take about 5 minutes. After that add the crushed garlic, let that cook for about 30 seconds or so, then turn the heat down, return the meat to the casserole and sprinkle in the flour. Using a wooden spoon, stir until all the flour has been absorbed into the juices. It will look rather stodgy and unpromising at this stage but not to worry - the long slow cooking will transform its appearance.
Now gradually stir in the beer and, when it's all in, let the whole thing gently come up to simmering point, and while that's happening add salt, freshly milled black pepper and the thyme and bay leaves. Then, just as it begins to bubble, put the lid on, transfer it to the centre shelf of the oven and leave it there for 2½ hours. Don't be tempted to taste it now or halfway through the cooking as it does take 2½ hours for the beer to mellow and become a luscious sauce.
Just before you want to serve the beef, preheat the grill, spread the croûtons with the mustard and sprinkle them with the grated gruyère, then arrange them on top of the meat and pop the casserole under the grill until the cheese is bubbling. Then serve straight away.