Preheat your oven to 400°F (200°C). Gently unroll the thawed puff pastry sheet and carefully press it into a 9-inch pie dish or tart pan. Trim any excess pastry, leaving a slight overhang, and crimp the edges if desired. Prick the bottom of the pastry several times with a fork.

Heat olive oil in a large skillet over medium heat. Add the diced onion and cook for 3-4 minutes until softened and translucent. Add the minced garlic and chopped spinach. Cook, stirring frequently, until the spinach has wilted completely and most of the liquid has evaporated, about 3-5 minutes. Remove from heat and set aside to cool slightly.

In a large mixing bowl, whisk together the eggs, milk (or half-and-half), salt, pepper, and optional nutmeg until well combined.

Stir the cooled spinach and onion mixture into the egg mixture. Fold in the crumbled feta cheese and shredded cheddar (if using). Pour the entire filling evenly into the prepared puff pastry crust.

Place the quiche in the preheated oven and bake for 25-30 minutes, or until the pastry is golden brown and puffed, and the center of the quiche is set (a knife inserted into the center should come out clean).

Remove the quiche from the oven and let it cool for at least 10 minutes before slicing and serving. This allows the filling to set completely.