See my substack article about mac and cheese here.

Peel and finely dice the celeriac. Don't throw the peel away - wash it, dry it and keep it available.

Bring a large pan of unsalted water to the boil then add the diced celeriac and the dried macaroni. Cook until just a tiny bit al-dente, then remove the pan from the heat but leave everything in the water overnight.

For the sauce, put the milk into a pan and add the bay leaves, onion, peppercorns, vegetable trimmings and the celeriac peel. Bring to the boil, remove from the heat and leave overnight.

Next day, wash and finely dice the tomatoes, add the salt and sugar, stir and set aside.

Drain the macaroni and celeriac, preserving the water. Blend the celeriac with just enough of the cooking water to achieve a texture that you like. Taste the celeriac purée and season to taste.

For the sauce, melt the butter and stir in the flour and mustard powder. Keep the heat very low and stir the roux until you're sure the flour is cooked out.

Strain the milk and add it very, very slowly, stirring constantly. You might not need all of the flavored milk, but it will thicken up, so don't stop adding milk until it's a little thinner than you want.

Still off the heat, stir in the grated Cheddar cheese. Taste and season if necessary.

Layer it to assemble. From the bottom you want celeriac purée, then macaroni cheese, then tomatoes, then celeriac purée, then macaroni cheese. Top with grated cheese and parmesan.

Bake in a pre-heated oven at 400°F for 35 - 45 minutes.