Prepare the sausage
Remove the skins from the sausages and crumble them roughly with your hands.

Cook the spinach
Pour a drizzle of extra virgin olive oil into a pan.
Once hot, add the frozen spinach cubes, distributing them evenly over the entire surface of the pan. It's important not to stack them: it's better if each cube touches the bottom so they cook uniformly.

Cover with a lid and cook over medium-low heat for about 10 minutes.

Add the sausage
Move the spinach to one side of the pan, freeing up half the surface.
Add the crumbled sausage to the free space and brown it over high heat for about 5 minutes.

Then, combine the sausage with the spinach, mixing well. Use a fork (or a wooden spoon, to avoid damaging the pan) to break up any large pieces of sausage.

Continue cooking over high heat until the mixture is well-drained: there should be no liquid left from the spinach, only a trace of oil. This step is crucial to prevent the base of the tart from becoming soggy.

Turn off the heat and let the filling cool completely.

Prepare the base
Lay the first puff pastry disc with its baking paper on a surface.

Spread the filling evenly over the entire surface, leaving about a 2 cm border.

Sprinkle the grated mozzarella over the top, distributing it well over the entire tart.

Close the tart
Cover with the second puff pastry disc.
Seal the edges well by folding both discs slightly inwards to seal.

Using a fork, prick the surface a few times.

Brush and bake
Beat the egg and brush it all over the surface.

Bake in a preheated static oven at 180°C for about 20 minutes, or until the tart is golden brown.