Beef Carbonnade (Flemish Beef Stewed in Beer)

Name: Beef Carbonnade (Flemish Beef Stewed in Beer)
Prep time: 40 min
Cook time: 3 h
Total time: 3 h 40 min
Yield: 6
Ingredients: 1 kg beef; 6 tbsp olive oil; 50 g butter; 6 onions; 3 tbsp plain flour; 900 ml dark beer; 1 tbsp sugar; 2 tsp white wine vinegar; 6 baguette slices; mustard, as needed; salt and pepper, as needed