For the mornay sauce. Prepare a bechamel sauce by heating the butter in a skillet over medium heat. When it has melted, add the flour, stir to incorporate and cook for a couple of minutes, without letting it take colorto form the roux.
Then add the milk, little by little, while stirring to prevent lumps from forming. Season to taste and season with a little ground white pepper and nutmeg. we cook during 10 minutesstirring constantly with a few rods until the sauce thickens.
Mix the cream with the egg yolk in a bowl and add it to the pan with the bechamel sauce. We continue beating with the rods while we cook the whole one more minute over low heat.
Remove the pan from the heat and add the grated cheese, mixing well. Once the cheese has melted and incorporated into the sauce, we rectify salt and pepper if necessary. We already have our sauce ready to use. We reserve,
Cut the pumpkin in half and remove the seeds. Season with salt and pepper, grease with a drizzle of oil and place on a baking tray lined with parchment paper. Add a little water and roast in the preheated oven at 200ºC, with heat above and below, for 30-45 minutes. When it is ready we empty it and reserve the skins.
Meanwhile, peel and finely chop the onion. Heat the extra virgin olive oil in a frying pan and cook over low heat. When it is translucent, add the mushrooms, finely chopped, and fry for a couple of minutes.
Then add the spinach and continue to sauté until be tender. Now all that remains is to add the roasted meat of the pumpkin, season with salt and pepper and sauté again to incorporate all the vegetables.
Place the pumpkin skins in a baking dish (Be careful, they are delicate), fill them with the previous mixture and cover them with mornay sauce. Gratin them under the grill until they acquire a nice golden color. We serve immediately.