Method

The night before, put the frozen spinach in a colander and thaw overnight in the refrigerator. The next day, gently press on the spinach to expel excess water. Set aside.

In a saucepot, melt the butter over medium heat. Add the onions and reduce the heat to a simmer. Sweat the onions gently on low flame until softened and nearly melting.

Add the flour to the pot and stir to incorporate. Cook this mixture for 5 minutes on low heat, stirring often to prevent burning.

Pour in the milk in stages, whisking constantly to incorporate the liquid before adding the next addition. Once all the milk is added, increase the heat to medium and bring the sauce to a simmer, stirring to prevent the bottom from burning.

Once the sauce begins to simmer, reduce the heat to low and cook for 5 minutes, or until the sauce no longer tastes of raw flour. Stir in the garum and cheese and season with the salt, pepper and nutmeg, to taste. Be generous with the nutmeg; it makes a big difference. Remove the pot from the heat.

Preheat the oven to 215C. Add the spinach to the pot of sauce and stir to incorporate. Pour this mixture into an appropriately-sized baking dish. Generously shower the top with more cheese. Bake for 10-15 minutes, or until the cheese is melted browned and the spinach mixture is bubbly. Remove from the oven and allow to stand for 10 minutes before serving.