It’s thickened by using a roux – cooked flour and butter.
Recipe for an authentic onion sauce
• 2 onions
• 1 tbsp flour
• 2 tbsps butter
• 1 tsp mustard
• 1 tsp tomato paste
• 120ml/½ cup red vermouth
• 240 ml/1 cup beef stock (made with water and a couple of beef stock cubes, or ideally buy real)
1. In a small saucepan melt 1 tbsp butter.
2. Peel and chop the onions and gently fry until they turn transparent.
3. Add a second tbsp of butter, and melt. Add the flour, mix well and cook gently for about ten minutes – you are making a roux.
4. Add the mustard and tomato paste. Mix in well.
5. Slowly at the vermouth and stock – very slowly at the beginning, adding just a little at a time and mixing the pasty onion mix in well to avoid lumps.
6. Simmer about twenty minutes, until the sauce is thick, unctuous and reduced to about two-thirds of its previous volume.