For the sauce: Heat a pan over medium heat and melt the butter. Add the garlic and fry until golden, about 1 minute. Add the cream, and then turn the heat down to medium-low. Keep stirring until the cream begins to steam; do not let boil. Add the Parmigianno, nutmeg, salt and some pepper, and stir until the cheese is melted and the sauce is thick; do not let boil. Add the spinach and stir until wilted. Remove from the heat.
For the oysters: Preheat the oven to 220°C. Heat a pan over medium-high heat. Add the pancetta and render until crispy. In a baking tray, lay down a bed of rock salt and place the oysters in their half shells atop the rock salt to stabilize them. Add 1 tablespoon of sauce per oyster. Sprinkle with the pancetta, Parmigianno, panko and drizzle with the olive oil. Bake until golden and bubbling, about 10 minutes.