2 onions, thinly sliced
mustard or horseradish sauce, to serve (optional)
1 tbsp plain flour
400ml/14fl oz beef stock (or chicken stock)
1 tsp dark soy sauce
1 tsp tomato pur茅e
knob of butter
4 slices emmental or mild cheddar (optional)
2 mini-baguettes, split horizontally
1 glass red wine (optional)
4 slices roast beef