Onion soup with Aceto Balsamico Tradizionale di Modena DOP Affinato - TONDO

Ingredients: 
1 kilogram of white onions
1 ½ litres of meat stock
10 grams of butter
120 grams of grated Gruyère or Fontina cheese
Stale French bread
2 tablespoons of white flour
Salt
Pepper
Aceto Balsamico Tradizionale di Modena DOP Affinato - TONDO

Slice the onions, put them in a saucepan where we have melted the butter, cover and cook them over a low heat, stirring often and taking care that they do not take on too much colour.
After about half an hour, sprinkle them with a spoonful of flour and, stirring constantly, gradually add a few ladles of boiling stock.
Cook until the onions are softened, then add all the remaining stock and continue cooking to reduce it a little.
Meanwhile, slice the bread, sprinkle it with a little grated cheese and toast it in the oven until the cheese has melted.
Place some of the bread slices in the bottom of 4 individual soup tureens, which can withstand the heat of the oven, and cover them with the onion soup.
Add the remaining bread and cheese on top and place the tureens in a hot oven to brown.
When a crispy golden crust has formed on the surface, remove from the oven and serve with a drizzle of Aceto Balsamico Tradizionale di Modena DOP Affinato - TONDO.