Preheat the oven to 375°F (190°C).
Place the pie crust in a 9-inch pie pan.
In a skillet, sauté the chopped onion over medium heat until translucent.
Add the chopped spinach to the skillet and cook until wilted.
In a bowl, mix together eggs, milk, salt, and pepper.
Spread the spinach and onion mixture evenly over the pie crust.
Sprinkle shredded cheddar cheese over the spinach.
Gently pour the egg mixture over the cheese and spinach.
Bake in the preheated oven for 45 minutes, or until the quiche is set and the top is golden brown.
Let the quiche cool for a few minutes before slicing and serving.