Veggie gravy

Name: Veggie gravy
Ingredients: 1 onion, chopped; 2 carrots, finely chopped (about 200g); 2 celery sticks, finely chopped (about 200g); 2 bay leaves; 1 large thyme sprig; large knob of butter (or 2 tbsp olive oil); 1 tbsp golden caster sugar; 2 tbsp plain flour; 2 tsp yeast extract (optional); 1 tbsp tomato purée; 2 tbsp balsamic vinegar; 1l vegetable stock; dark soy sauce, to season (optional