In a food processor, pulse flour, butter, salt, and icing sugar until crumbs. Add yolk, then a little water if dry. Wrap, chill 30 min.
Roll out to 3 mm, line a 23 cm tart tin, chill again 20 min. Blind bake at 180 °C (160 °C fan) for 15 min with baking beans, then 8–10 min without until golden.
Simmer cherries with 100 ml water until soft. Push through sieve to remove pits/skins You should end up with 400–500 g purée.
Heat purée with sugar. Stir in starch slurry and lemon juice, cook until thick and glossy. Whisk vigorously while cooking the starch out to avoid burning.
In a bowl, whisk whole egg + yolks. Whisk in a ladle of the hot cherry mix and return everything to pan. Cook gently until custardy. Off heat, whisk in butter. Pour into pastry case.
Beat egg whites with lemon juice until you reach soft peak. Add sugar 1 tbsp at a time, whisking to stiff glossy peaks.
Spread or pipe meringue over hot curd, sealing edges. Use a spoon to lift the meringue as you like. Bake 15–20 min at 180 °C (160 °C fan) until golden.
Cool to room temp, then chill at least 3–4 hrs before slicing.