Roast the celeriac

1. Preheat the oven to 190ºC (170ºC fan). Line a baking tray with parchment paper.
2. Thoroughly scrub the celeriac and trim away any hairy roots from the base. Using a fork, pierce the celeriac all over. Place into a large bowl with half of the olive oil and the kosher salt, then toss well to ensure it is completely coated. Transfer to the prepared baking tray.
3. Roast for 2½ hours, basting with the olive oil from the tray every 30 minutes, until the celeriac is deeply browned all over and a butter knife slides in easily to the centre.
4. Allow the celeriac to cool slightly until safe to handle, then cut into thick slices. Place under a hot grill for a few minutes on each side until lightly golden and caramelised around the edges.

Make the dill & Dijon sauce

1.  Lightly toast the ground coriander and fennel in a dry pan over medium heat for 30 seconds until fragrant, then let cool.
2. Add spices to a bowl with the white wine vinegar and leave to bloom for 1–2 minutes. Whisk in the Dijon mustard and lemon juice.
3. Slowly pour in the remaining olive oil while whisking continuously until emulsified. Fold through the chopped dill and season with flaky sea salt to taste. Spoon over the warm celeriac to serve.