Preheat the oven to gas 6, 200°C, fan 180°C. With a little flour, roll the pastry out to roughly fit a 36x27cm (14x11in) baking tray. Chill in the fridge.
Pan-fry the bacon until it's crisp. Drain on kitchen paper. Thinly slice the leeks, using some of the green tops as well. Add to the bacon pan along with the butter. Season. Sauté for 5 minutes or until softened.
In a bowl, mix the egg yolks with the crème fraîche. Season. Sprinkle the grated Gruyère over the pastry, then top with the bacon and leeks. Pour the cream mixture over the tart, scatter with the onion slices and season with black pepper.
Bake for 20 minutes until the pastry is crisp. Serve with a dressed green salad.
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