Wash the potatoes, then rub with sea salt. Prick the skins with a fork and place on one side of a baking tray, then bake in the hot oven for 30 minutes.
Cut the celeriac into large chunks and separate the garlic bulb into cloves, then place onto a large piece of baking parchment. Sprinkle with sea salt and black pepper, drizzle over 2 tablespoons of olive oil, toss to coat, then wrap up into a loose parcel.
Once the potatoes have had 30 minutes in the oven, add the celeriac parcel to the tray and bake for a further 20 to 30 minutes, or until the potatoes and celeriac are soft and cooked through.
Scoop the potato flesh from the skins, squeeze out the garlic flesh and mash together with the celeriac. Pick in the thyme leaves and add the rest of the oil and mash again. Taste, season and serve.