Heat your oven to 200°C/Fan 180°C/Gas 6. Fill the kettle and boil it. Halve the celeriac. Slice off the peel and roots on the diagonal, to leave as much celeriac behind as possible. Rinse and chop into 2-3cm-wide chunks.

Pop the celeriac in a medium pan. Pour in enough boiling water to come halfway up the veg. Add a good pinch of salt and pepper. Bring to the boil. Cover and simmer for 20 mins, or till tender when pierced with a fork. Remove from the heat. Set aside, lid on, for 5 mins to steam the celeriac.

While the celeriac cooks, whisk 1 tbsp each Dijon mustard and olive oil with 2 tbsp cold water and a pinch of salt and pepper to make a dressing.

Pop the cod fillets into a small baking dish, skin-side up. Use a pastry brush or the back of a teaspoon to brush half the mustard dressing over the top of the fillets (keep the other half for later).

Peel and finely chop the garlic. Finely chop half the thyme leaves. Scatter the chopped garlic and thyme over the cod with a pinch of salt and pepper. Scatter over the breadcrumbs.

Trim the roots and dry ends off the spring onion and finely slice it. Arrange the spring onion around the cod and pop the dish into the oven for 10 mins, till the cod is cooked through, is white and flakes easily.

While the cod cooks, toss the rocket with the remaining mustard dressing. Core and thinly slice 1 apple. Add to the rocket, toss and set aside.

Drain any excess water from the celeriac. Tip into a processor. Peel, core and chop the remaining apple. Add to the celeriac with 2 tbsp crème fraîche and blend till smooth. No processor? Tip the celeriac back into the pan, add the apple and use a masher to crush everything, then stir in the crème fraîche.

Lift the spring onion out of the cod dish and stir it into the celeriac mash, along with the remaining thyme leaves. Taste and season if needed. Serve the cod with the champ and salad on the side.