Double baked cheese soufflés

Name: Double baked cheese soufflés
Ingredients: 1 small onion, peeled, cut into wedges; 275ml/10fl oz whole milk, plus extra for topping up if needed; 1 bay leaf; 40g/1½oz butter, plus extra for greasing; 40g/1½oz plain flour; 100g/3½oz mature cheddar, coarsely grated; 1 tsp english mustard (or 2 tsp dijon mustard); Freshly grated nutmeg, to taste; Salt and freshly ground black pepper; 3 free-range eggs, separated; 1 heaped tbsp chopped chives, plus extra for garnish; 50g gruyere or mature cheddar, finely grated; 6 tbsp double cream