Preheat the oven to 375°F (190°C).
Place the pie crust into a 9-inch pie dish, crimp the edges, and set aside.(Optional: Pre-bake the crust for 10 minutes for extra crispness.)
In a skillet over medium heat, melt the butter (or heat the olive oil). Sauté the diced onion until softened and translucent, about 5 minutes.
Add the chopped spinach and cook until just wilted, about 2–3 minutes. Remove from heat and allow to cool slightly.
In a mixing bowl, whisk together the eggs and milk until fully combined. Season with salt and pepper.
Spread the spinach and onion mixture evenly over the bottom of the pie crust. Sprinkle the chopped bacon over the spinach.
Evenly distribute the shredded Cheddar cheese on top.
Carefully pour the egg mixture over the filling.
Place the quiche on a baking sheet to catch any spills. Bake for 40–45 minutes, or until the top is golden and the center is set.
Remove from the oven and let cool for 10–15 minutes before slicing and serving.