Line the tin: Roll the pastry on a floured surface to about 3mm and use it to line a 20cm loose-bottomed tart tin, leaving a little overhang. Chill for 30 minutes.
Blind bake: Heat the oven to 190C (170C fan, 375F). Line the shell with foil and baking beans, then bake 15 to 20 minutes until set and pale gold. Lift out the foil and beans and bake another 5 minutes until golden. Trim the overhang level. Drop the oven to 135C (115C fan, 275F).
Make the filling: Whisk the lemon juice, sugar, egg yolks and cream together gently, taking care not to whip in air.
Pour and bake: Set the tin on the oven shelf, then pull the shelf half out. Pour the filling into the shell, slide the shelf gently back, and bake about 30 minutes until just set with a faint wobble.
Cool slowly: Turn the oven off and leave the tart inside to cool, so it carries on setting. Take it out only when fully cooled, after a few hours.
Meringue, to serve: Warm the sugar in a low oven for a few minutes. Whisk the egg whites to stiff peaks, then add the warm sugar a spoonful at a time, beating to a firm, glossy meringue. Pile onto the tart, swirl into peaks, and torch or grill briefly until golden.
