Heat the oven to 200c
Bring the potatoes and celeriac to the boil in salted water and boil for 5mins until just tender but not soft.
Leave to cool and then grate coarsely into a large bowl.
Fry the bacon in a splash of olive oil for 5mins until crispy then tip into the bowl with the potatoes and celeriac.
Mix in the gruyere, thyme leaves and butter and stir thoroughly, season generously with salt & pepper.
Heat a knob of butter and a splash of olive oil in a non stick frying pan until sizzling and then tip in the potato mix (divide this in quarters if you are making individual rosti).
Gently pat down the mixture with a spatula to flatten and turn heat down to a gentle flame.
Cook for 10-15mins (8-10mins for individual rosti) shaking pan from time to time to loosen.
Flip the rosti onto a plate and slide back into the pan to cook for the same again on the other side.
Make a hollow in the top of the rosti and break a quail egg into it. Finish off in the oven for 1 min until the yolk has just turned opaque.
Note: the best way to crack a quail egg is with a small sharp knife. Gently tap the shell and then pierce membrane beneath. Drop the egg into a little ramekin before using – this way any unwanted broken yolks can be avoided!