Caramelised Onion Tart Recipe

Ingredients

• 200 grams all-purpose flour
• 100 grams unsalted butter
• 3 tablespoons cold water
• 2 tablespoons olive oil
• 4 medium onions (thinly sliced)
• 1 tablespoon balsamic vinegar
• 1 tablespoon brown sugar
• 1 teaspoon fresh thyme leaves
• 1 teaspoon salt
• 0.5 teaspoon black pepper
• 100 grams crème fraîche
• 100 grams grated Gruyère cheese
• 1 unit egg yolk