Method:
Preheat the oven to 350°F. Remove piecrust from the freezer and thaw for ten minutes. Prick the bottom with a fork and bake for 7 to 10 minutes, until lightly browned. Remove crust from the oven to cool.
Heat olive oil in a medium sauté pan. Add garlic, spinach, salt, pepper and sage. Cook, stirring constantly, for about 1 minute, or until the spinach is just wilted. Stir in pine nuts.
In a small bowl, mix mozzarella and Parmesan cheeses. In another bowl, whisk milk and eggs together.
Sprinkle 1/4 cup of the cheese mixture on the piecrust. Top with half of the spinach mixture.
Place potato slices on top of spinach. Add half remaining cheese mixture and then remaining spinach mixture. Top with remaining cheese.
Place the pie pan on a sheet pan. Carefully pour in milk mixture. Bake for 45 to 50 minutes, until a knife inserted into the center comes out clean. Cool slightly before cutting.