Gruyère and prosciutto sour cream tart | Gourmet Traveller

Ingredients

• 1½ tbsp olive oil
• 2 onions, finely chopped
• 150 gm piece prosciutto, finely chopped, plus 8 thin slices
• 1 tsp chopped thyme
• 11 eggs, at room temperature
• 500 gm sour cream
• 300 ml pouring cream
• 200 gm Gruyère, coarsely grated
• Pastry
• 180 gm butter, finely chopped
• 350 gm plain flour (2 1/3 cups)
• 1 egg yolk