Preheat oven to gas 4, 180°C fan, 160°C. On a lightly floured surface, roll the pastry sheet out until it is a few centimeters larger all around. Trim the edges, cut into 6 even squares, then use each square to line each hollow in a large, 6-hole muffin tin, or 6 individual ramekins. Don't worry if the corners stick up or look untidy. Chill for 20 minutes.
Line each pastry cup with a square of nonstick baking paper and fill with baking beans or dried pulses. Bake for 10 minutes then carefully remove the paper and beans and return to the oven for a further 5 minutes, until the pastry is just golden.
Meanwhile, lay 6 of the bacon rashers on a baking tray and bake for 10 minutes and set aside.
Chop the remaining 2 rashers of bacon and gently fry with the onion in the olive oil for 10 minutes, until onion has softened but not coloured. Transfer to a bowl and stir in the sage, cream and beaten egg and season. Tuck a bacon rasher into each pastry cup, curling it around to fit. Divide the onion and cream mixture between the tarts and place one whole egg yolk in each. Bake for 10-15 minutes, until the pastry is golden and the bacon rashers are just brown.
Set aside to cool for 10 minutes before turning out, then grind with a little pepper and serve with ketchup, if desired.
Find out more about Vintage Farmhouse Cheddar.
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For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home.
If serving vulnerable groups, elderly people, toddlers, pregnant women and people who are unwell, cook eggs until the whites and yolks are solid.