French Onion Soup - Pinch Of Nom

Ingredients

• low-calorie cooking spray
• 2 red onions, (peeled and finely sliced)
• 2 brown onions, (peeled and finely sliced)
• 3 garlic cloves, (peeled and finely sliced)
• 1 tbsp balsamic vinegar
• 1 tbsp Henderson's relish (or Worcestershire sauce)
• 1 litre boiling water
• 1 vegetable stock pot
• 2 beef stock cubes