For the celeriac, preheat the oven to 190C/350F/Gas 5.
Place the celeriac into a roasting tray.
Mix the egg whites, table salt and rosemary together in a large bowl until the mixture forms a paste.
Cover the celeriac in a 2cm/¾in thick layer of the salt paste, ensuring there are no gaps. Bake in the oven for 45-50 minutes.
Meanwhile, for the grouse, lay two rashers of smoked bacon over the fat part of each grouse breast. Tie the bacon to the birds using kitchen string.
Heat the oil in a heavy-based flameproof casserole over a medium heat. Once the oil is hot, add the grouse and fry, turning regularly until browned on all sides. Remove from the pan and set aside.
Add the shallots, garlic and thyme leaves to the same casserole and fry for 1-2 minutes, or until softened.
Add the blackberries and chicken stock and place the grouse on top. Cover the casserole with the lid and cook in the oven for 10-12 minutes, or until the grouse are cooked through.
Once cooked, remove the casserole from the oven and set aside for 10 minutes.
Remove the birds from the casserole and set aside. Keep warm.
Bring the remaining contents of the casserole to the boil on the hob. Reduce the heat until the mixture is simmering. Continue to simmer until the volume of liquid has reduced by half - about five minutes. Whisk in the butter until the gravy is glossy. Keep warm.
When the salt-baked celeriac has cooked, remove it from the oven, chip away the salt crust and scoop out the baked celeriac into a bowl.
Add a splash of olive oil to the celeriac, then season, to taste, with sea salt flakes and freshly ground black pepper. Mix the celeriac with a fork until combined.
To serve, remove the string from the grouse, cut the breasts from the carcasses and cut any sinew from the breasts. Divide the salt-baked celeriac equally between two serving plates. Place one grouse breast on top of each portion of celeriac. Drizzle the gravy around the edge of the plates.