Pissaladiére

1x 320g sheet of all-butter puff pastry

3 tbsp golden rapeseed oil

25g butter

850g white onion, finely sliced

2 cloves garlic, minced

3 sprigs thyme (1 sprig reserved for baking)

16 anchovy fillets

12 black pitted olives

Freshly cracked black pepper

1 egg yolk