Rare: 4 minutes on each side Med-rare: 5 minutes on each side Medium: 6 minutes on each side (recommended) Well-done: Not recommended
Pre-heat the oven and a baking tray to 200°C/390°F.
To make the caramelised onions, heat the butter and oil in a saucepan over a medium heat. Add the onions and a big pinch of salt and cook slowly for about 10 minutes, stirring occasionally to prevent them from catching on the bottom of the pan.
When the onions are soft and turning golden brown, stir in the sugar and balsamic vinegar and cook on a low heat for a further 5 to 10 minutes until sticky and caramelised.
On a lightly floured surface, use a rolling pin to roll out the puff pastry into a large rectangle.
Using a sharp knife, score a 2cm border around the edge of the pastry, going almost but not quite through the pastry, then glaze the crust edge of the pastry with the beaten egg yolk.
Evenly spread the onions and cheeses over the pastry so that they meet the edge of the dough and sprinkle over the thyme sprigs and tarragon leaves. Season with salt and pepper.
Bake the tart on the pre-heated baking tray for about 20 minutes or until the pastry is just crisp and turning golden.
Whilst the tart is crisping up, lightly coat the rump steak in olive oil and generously season with salt and freshly ground black pepper.
Grill the ribeye in a smoking hot griddle pan, turning to cook on both sides to your preferred doneness (see cooking guideline above) and leave the meat to rest for half its cooking time before slicing it into slivers.
Top the tart with the slices of juicy ribeye steak, slice into squares and serve whilst hot!