Boil the stock and whisk in the crème fraîche, mustard, salt, mustard powder, and lemon juice.
Sieve the sauce into a jug and set aside.
Boil the bacon for 20 minutes, drain, and chop into lardons.
Boil the onions for 7 minutes, refresh in cold water, and halve.
Preheat the oven to 200°C.
Roll pastry to 4mm thick, cut into four squares, and cut two holes in each.
Place squares on a tray, brush with egg yolk, and chill for 15 minutes.
Bake the pastry for 20 minutes until golden.
Turn the grill to high.
Oil and season the sardine fillets and heads, then grill for 3 minutes.
Fry the bacon in butter for 4 minutes, add onions, and stir in enough sauce to coat.
Bring water to a boil with vinegar and salt, then reduce to a simmer.
Poach the quail eggs in a whirlpool of water for 2 minutes.
Assemble the fish and sauce, blend the remaining sauce until frothy, and top with pastry and heads.