1. Melt the butter in a large, heavy saucepan and slowly brown the onions. This should take about 1 hour, the onions will become super soft and greatly reduced in quantity. Add the garlic after 45 minutes.
2. Stir in the flour and cook for 1 minute. Add the stock, salt and pepper and stir until boiling then simmer for 20-30 minutes.
3. Pre-heat the oven to 200oc/400of/ Gas mark 6.
4. Mix the Gruyere with the mustard and some pepper. Spread this on the bread slices and place them on the bottom of an earthenware tureen.
5. Pour over the soup, the bread will rise to the top. Place in the oven until well browned and bubbling.
Wash it down with a nice glass of Beaujolais - it’s a meal in itself.