Marmite carbonnade

Name: Marmite carbonnade
Ingredients: 85g unsalted butter, plus extra for spreading; 4 onions, halved and sliced; 3 garlic cloves, peeled and smashed; 1.5kg/3lb 5oz stewing steak, cut into large cubes; 850ml beef stock; 1½ tbsp plain flour; 400ml can stout; about 10 thyme sprigs, tied into a bunch; 2 bay leaves; 1 tbsp light muscovado sugar; 2 tsp red wine vinegar; 2-4 tbsp mushroom ketchup (optional, see Know-how, below); 1 celery heart, trimmed and sliced; about 2 tsp Marmite; about 25 thin slices French bread (about ¾ baguette - day-old is fine)