Heat the oven to 180° fan. Cut the celeriac into 1.5cm cubes and toss in the oil, with salt and pepper. Roast on a baking tray for 25-30 mins. Prepare a 18cm springform cake tin by oiling all the inside surfaces.
Meanwhile, melt the butter in a pan, stir in the flour and cook for a minute. Take off the heat and whisk in the milk slowly to make a smooth sauce. Return to the heat and simmer for a few minutes to thicken, though note that it will be quite a thin sauce. Allow to cool.
Rinse the spinach and rocket and gently cook in a lidded saucepan until it has all wilted. Drain the leaves as much as you can, then roughly chop.
Combine in a large bowl the eggs, ricotta, 75g of the parmesan and a generous grating of nutmeg and a good grind of black pepper, then pour in the cooled sauce. Add the chopped leaves and mix well, then stir in three-quarters of the roasted celeriac.
Pour the mixture into the cake tin and level the top. Sprinkle over the remaining 25g parmesan. Bake at 180° fan for 35-40 minutes. Allow 10 minutes of cooling in the tin before opening the springform tin and placing the base of the tin onto a plate for serving. Top with the toasted pine nuts, the remaining celeriac and the reserved rocket leaves.