Onion soup with gruyère twists

Ingredients

• 75 gram butter
• 2 large onions, sliced
• 1 leek, sliced
• 1 red onion, sliced
• 4 shallots, sliced
• 3 garlic cloves, crushed
• 2 teaspoon thyme leaves
• 6 cup vegetable stock
• 300 gram potatoes, peeled, chopped
• 3 spring onions, sliced
• crème fraïche, snipped chives, fried shallots to serve
• Gruyère twists
• 2 sheets frozen puff pastry, partially thawed
• 1 tablespoon wholegrain mustard
• 1/3 cup grated gruyère