Balsamic Onion Tart with Goat Cheese and Thyme

INGREDIENTS

• Crust:
• 2 cups (225g) all-purpose flour
• Pinch salt
• ½ teaspoon mustard powder
• Pinch cayenne pepper
• 115g butter
• 4 tablespoons iced cold water
• Filling
• 2 tablespoons butter
• 4 white or yellow onions, very finely sliced
• 1/3 cup balsamic vinegar
• 1/4 brown sugar
• Salt and freshly ground black pepper
• Stem of fresh thyme
• 5 oz. soft goat cheese
• 2 tablespoons fresh thyme leaves or sprigs of fresh thyme