Goat's cheese, potato & onion tart

INGREDIENTS

• 250g pack shortcrust pastry , fresh or frozen
• 25g butter
• 1 tbsp olive oil
• 2 onions , sliced
• 1 medium potato , peeled and sliced
• 2 garlic cloves , crushed
• 2 tbsp fresh thyme leaves
• 140g goat's cheese , broken into pieces
• 3 eggs
• 200g carton crème fraîche , half fat is fine
• 100g bag rocket to serve, optional