Melt butter in a medium sized saucepan, then stir in the flour to form a paste cooking for just 1 minute until a thick paste forms then remove from the heat and add the milk and stir until it thickens 2-3 minutes, turning the heat back on if needed. Cool the mixture completely.

Put a frying pan on a low heat and gently cook the spinach and arugula until wilted, about 3 minutes then drain of any extra moisture.

Once the butter-flour-milk mixture has cooled, stir in the ricotta, eggs, seasonings, and parmesan cheese (reserve a little cheese for the top). Next, mix in the roast celeriac and greens then pour the mixture into the prepared baking tin (I used a buttered and breadcrumb lined cake pan) and sprinkle over the remaining cheese on top.

Bake for 35-40 minutes until the top browns and the middle of the sformato is set. Cool for at least 10 minutes before removing for the tin to serve with pinenuts or almonds on top (if desired).