Grease a 23x10cm/9x4in terrine mould with butter.
For the pastry, mix the flours and salt in a bowl.
In a small saucepan, bring 175ml/6fl oz water, the butter and lard to the boil slowly, over a low heat.
When the fats have melted, pour the mixture over the flour and stir until it forms a soft dough.
Turn the dough out onto a lightly floured work surface and knead very gently until smooth and slightly cooled. Set one-third of the dough aside.
Roll out the remaining pastry until it is large enough to fit inside the terrine mould.
Line the terrine mould with the pastry, pressing it into each corner and pushing it until it reaches all the way up the sides. Chill in the fridge until needed.
Preheat the oven to 180C/160C Fan/Gas 4.
For the filling, mix together all the filling ingredients in a bowl until well combined. Spoon the mixture into the pastry case, pressing it gently into the corners to make sure there are no gaps.
Roll out the remaining pastry to form a lid large enough to cover the terrine.
Brush the edges of the pastry case with some of the beaten egg. Lay the pastry lid on top and crimp together at the edges. Decorate the top of the terrine with any remaining off-cuts of pastry, then brush it with the remaining beaten egg. Pierce a hole in the pastry lid to allow the steam to escape.
Bake the terrine in the oven for 1-1¼ hours, or until the pastry is crisp and golden-brown and the filling is cooked through.
Meanwhile, for the apricot and tomato chutney, heat a large, heavy-based frying pan over a medium heat, add the sugar and heat until it has completely melted and is starting to caramelise.
Stir in the onion, tomatoes, apricots and dried chilli flakes, then pour over the vinegar to deglaze the pan – the mixture will become lumpy, but continue to cook it until the sugar dissolves and the mixture becomes smooth.
Bring the chutney mixture gently to the boil and let it bubble for 5-10 minutes, or until the vegetables are tender and the mixture has thickened slightly. Season, to taste, with salt and freshly ground black pepper, then decant into a sterilised jar and seal. (The chutney will keep for up to two weeks in a sealed, sterilised jar.)
To serve, remove the terrine carefully from the mould and serve immediately, or chill in the fridge and serve cold. Serve in slices with a dollop of chutney and some sliced radishes alongside.