Preheat oven to 375°F (190°C).
Blind bake the pastry shell for 10 minutes weighted with pie beads, then remove and let cool.
Sauté spinach and minced garlic in a skillet until wilted; extract all excess liquid by squeezing through a clean kitchen towel.
In a large bowl, whisk eggs, heavy cream, and ricotta until the consistency is uniform.
Stir in the dry spinach, Parmesan, Gruyère, salt, pepper, and nutmeg.
Transfer the custard mixture into the par-baked pastry shell, spreading evenly.
Bake for 35 to 40 minutes until the custard has set and the surface is golden.
Rest the quiche for 10 minutes at room temperature to allow the structure to firm before slicing.