1) Bring the stock up to the boil and allow to reduce by half.
2) Whisk ingredients into the creme fraiche: both mustards, a pinch of salt and a squeeze of lemon.
3) Bring the sauce back up to the simmer and taste to check for the right amount of mustard and seasoning.
4) Pass through a sieve and keep warm to one side.
1) Roll out the puff pastry to 3-4mm thick and let rest in the fridge. Once chilled, cut out four square shapes, with two of the corners cut out with a round cutter the size of a golf ball. Brush with the egg yolk and chill once again in the fridge.
2) Once the pastry is chilled, bake on a tray for about 20 minutes or until a deep golden brown.
3) Fillet and portion the sardines. Arrange them on a tray, along with the heads and tails, for grilling, coating the fish with rapeseed oil, salt and pepper.
4) Take the bacon and cut into pieces (lardons), fry them gently in a little butter to release flavour.
5) While the bacon is frying, poach the quail eggs. Have ready a simmering pan of water with a tablespoon of vinegar and a pinch of salt. Crack the eggs into a little ice-cold water.
6) Pour off any excess cold water (there should only be enough to just cover) then swirl the simmering water and gently pour the eggs into the pan. Let cook for a minute before removing.
7) Grill the sardines. Once the bacon is lightly fried, cut the onions in half and add them to the bacon, along with a couple of spoons of the sauce. Turn up the heat on the pan to reduce the sauce so it coats the bacon and onions.
To assemble the dish, place the onions and bacon on the bottom, then arrange the sardine fillets on top, keeping the head and tails to finish. Place the eggs around the fillets, hand blend the sauce and pour over the bubbles (to keep the dish light). Top with the puff pastry, poke the head and tails of the sardines out of the corners and serve.