Preheat your oven to 375°F (190°C). Press the pie crust into a 9-inch pie dish and crimp the edges. Prick the bottom with a fork, line with parchment and pie weights, and blind bake for 10 minutes. Remove weights and bake 5 more minutes until lightly golden.

Melt the butter in a large skillet over medium heat. Add the diced onion and cook for 3-4 minutes until softened. Add the garlic and cook for 30 seconds.

Add the spinach to the skillet in batches, stirring until wilted, about 2-3 minutes. Transfer to a colander and press out as much moisture as possible. Roughly chop the spinach.

In a large bowl, whisk together the eggs, heavy cream, milk, salt, pepper, and nutmeg until smooth.

Spread the chopped spinach and onion mixture evenly over the bottom of the par-baked crust. Sprinkle the shredded Gruyere cheese over the spinach.

Pour the egg custard mixture over the spinach and cheese, filling the crust almost to the top.

Bake for 40-45 minutes until the quiche is set in the center and the top is lightly golden. A knife inserted in the center should come out clean.

Let the quiche cool for at least 10-15 minutes before slicing. This allows the custard to set properly. Serve warm or at room temperature.