French Onion Soup- Justine Pattison's low calorie but delicious recipe

Ingredients

• 2 tsp sunflower oil
• 6 medium onions, (thinly sliced)
• 1 tbsp fresh thyme leaves, (plus extra to garnish)
• 2 tbsp plain flour
• 1.5 litres hot beef stock ((made with 2 stock cubes))
• 12 thin slices of baguette ((about 15g per slice), cut diagonally)
• 50 g Gruyere cheese, (finely grated)
• flaked sea salt
• ground black pepper