Preheat the oven to 190°C.
Place the celeriac into a roasting tray.
Mix egg whites, table salt, and rosemary into a paste.
Cover the celeriac in a 2cm thick salt layer and bake for 45-50 minutes.
Tie two bacon rashers over each grouse breast with kitchen string.
Brown the grouse in oil in a flameproof casserole, then remove.
Soften shallots, garlic, and thyme in the same casserole for 1-2 minutes.
Add blackberries and stock, return grouse to the pan, cover, and bake for 10-12 minutes.
Remove the casserole from the oven and let it rest for 10 minutes.
Extract the grouse and keep them warm.
Boil the remaining liquid until reduced by half, then whisk in butter.
Chip the salt crust off the celeriac and scoop the flesh into a bowl.
Add olive oil and seasoning to the celeriac and mash with a fork.
Serve grouse breasts over the celeriac mash drizzled with blackberry gravy.