For the mustard sauce
- 250ml/9fl oz white chicken stock
- 125g/4½oz crème fraîche
- 25g/1oz English mustard
- pinch salt
- ½ tsp mustard powder
- squeeze lemon juice
For the pie
For the mustard sauce, bring the stock to the boil in a non-reactive saucepan. Whisk in the crème fraîche, mustard, salt, mustard powder and lemon juice until well combined. Bring back to the simmer.
Pass the sauce through a fine sieve into a jug and set aside.
For the pie, cook the bacon in boiling water for 20 minutes. Drain, then allow to cool slightly before chopping into lardons.
Bring another pan of water to the boil and cook the baby onions for 6-7 minutes, or until tender. Drain and refresh in cold water, then slice each onion in half. Set aside.
Preheat the oven to 200C/400F/Gas 6.
Roll out the puff pastry until 3-4mm thick, then cut into 4 equal-sized squares. Using a small circular pastry cutter the size of a golf ball, cut out 2 holes in each pastry square.
Place each square on a baking tray and brush with the beaten egg yolk. Chill in the fridge for 15 minutes.
Bake the pastry squares in the oven for 18-20 minutes, or until golden-brown and crisp. Remove from the oven and set aside.
Turn the grill on to high.
Place the sardine fillets, heads and tails on a solid grill tray, brush with the oil and season with salt and freshly ground black pepper. Grill for 2-3 minutes, or until golden-brown and just cooked through (the fish should be opaque all the way through and flake easily).
Heat a frying pan until medium hot, add the butter and bacon lardons and fry gently for 3-4 minutes, or until golden-brown. Add the onions and stir in enough sauce to coat all the ingredients in the pan. Reserve the remaining sauce and keep warm.
Bring a small pan of water to the boil, add the vinegar and a pinch of salt. Reduce the heat to a simmer.
Crack the quail's eggs into a small bowl of iced water, then pour off any excess (there should only be just enough water to cover the eggs). Swirl the simmering water with a wooden spoon to create a whirlpool effect, then gently pour the quails' eggs into the centre of the whirlpool. Poach for about 1-2 minutes, or until the egg whites have set and the yolk is still runny. Remove with a slotted spoon and drain on kitchen paper.
To serve, divide the onion and bacon mixture between 4 serving plates. Arrange the sardine fillets on top, then place four poached quails' eggs around the fillet. Using a stick blender, blend the remaining sauce until frothy. Spoon the froth over the top of the sardines and eggs. Top each pile with the puff pastry squares, then place the sardine heads and tails through each hole in the pastry. Serve immediately.