Heat the oven to 180 C / 355 F. Butter a 6-cup loaf pan or 4 individual 180 ml ramekins generously, then dust the inside surfaces with finely grated Parmigiano. Tap out any excess.

Place the buttered mold(s) inside a deep roasting pan. Set aside while you prepare the filling.

If using fresh spinach, wash it and place it in a large pan over medium heat with only the water clinging to the leaves. Cover and cook for 3 to 4 minutes, stirring once, until wilted and tender.

Drain the spinach through a colander and press firmly with the back of a spoon. Once cool enough to handle, wrap it in a clean kitchen towel and squeeze hard until no more water comes out.

Chop the squeezed spinach finely. Melt 15 g butter in a small pan over medium heat, add the crushed garlic clove, and cook 1 minute until fragrant. Add the chopped spinach, stir to coat, and cook 2 minutes. Remove the garlic and season with salt and pepper.

Melt 30 g butter in a medium saucepan over medium-low heat. Add the flour and stir with a wooden spoon for 1 to 2 minutes until the mixture turns pale and smells faintly nutty.

Pour in the warm milk in a steady stream, whisking constantly to prevent lumps. Continue stirring over medium-low heat for 4 to 5 minutes until the sauce thickens to a consistency that coats a spoon slowly. Season with salt and nutmeg. Remove from heat.

Stir 2 tablespoons of the warm bechamel into the beaten eggs to temper them, then pour the egg mixture back into the saucepan with the remaining bechamel. Mix well.

Add the cooked spinach and grated Parmigiano to the bechamel-egg mixture. Stir until evenly combined. Taste and adjust salt.

Spoon the mixture into the prepared mold(s), smoothing the surface flat with a spatula.

Pour enough boiling water into the roasting pan to come halfway up the sides of the mold(s). Carefully transfer the roasting pan to the oven.

Bake for 35 to 40 minutes for a large loaf pan, or 25 to 28 minutes for individual ramekins, until a skewer inserted in the center comes out clean and the top is pale golden.

Remove from the bain-marie and rest the sformato in the mold for 5 minutes. Run a thin knife around the edge, place a serving plate face-down on top, and invert firmly. Serve warm.