Heat the oil and butter together in a saucepan until melted. Add the finely sliced onions, put the lid on the pan and sweat them down over a medium heat. Cook the onions until they are golden and soft but not burnt.
Add the consomme or stock. Bring to the boil and simmer over a low heat for 30 minutes, until the liquid is reduced and thickened slightly.
Cut the bread into two small circles with a biscuit cutter. Toast one side lightly under the grill, remove the grill pan and turn the circles over. Spread the untoasted side with the mustard and top with grated gruyere. Return the toast to the grill until the cheese is golden and bubbly.
Serve the soup into your bowl and top with the cheesy toast.
Use a good beef stock
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