Step 1

Heat the oven to 180C/fan 160C/gas 4 and butter and line a deep 23cm springform cake tin.

Step 2

Cook the potatoes in a large pan of lightly salted boiling water for 8-10 minutes or until tender to the point of a knife. Scoop out with a slotted spoon, then add the asparagus for 2 minutes and drain well. Cut the potatoes into 1-2cm slices.

Step 3

Heat the butter with the onion and a pinch of salt, and cook for 5 minutes until softened. Add the flour and cook for 2-3 minutes until smelling toasty. Add the milk, a little at a time, while whisking, adding more once fully incorporated. Simmer for 5 minutes until thickened, then stir in the spinach, nutmeg, mustard, Italian hard cheese and lots of seasoning. Cool for 5 minutes, then stir in the asparagus, new potatoes and peas. Crack in the eggs and whisk well, then pour into the prepared tin and put into the oven for 45-50 minutes or until cooked through.

Step 4

Put the onions into a bowl with a pinch of salt and the vinegar and use your hands to scrunch them until they soften slightly.

Step 5

Cool the sformato for 10 minutes in the tin, then remove the sides, invert onto a plate and cool to room temperature. Stir the parsley into the onions, drain, scatter over the top of the sformato, cut into wedges and serve.