Prepare your pie dish by brushing with olive oil, spraying with pan spray, or greasing with a bit of butter.
Preheat the oven to 375 degrees Fahrenheit. Place your pie crust in the dish. Once the oven is up to temperature, poke the pie crust all over with a fork, then bake for 10 minutes. Set aside to cool.
Brown the sausage over medium heat in a frying pan with 1 tablespoon olive oil for about 10 minutes. Once fully cooked, remove from the pan.
Add the garlic to the pan and cook until fragrant, 30 seconds to 1 minute. Add the spinach and cook until fully wilted, 2 to 5 minutes.
Add the eggs, milk, rosemary, salt, pepper, sausage, spinach, cheese, and garlic to a large bowl and whisk together.
Pour the egg mixture into the cooled pie crust and place in the oven to bake for 35 to 45 minutes. Be sure not to overfill or your egg may spill over while cooking.
Quiche is done when the center of the egg mixture is just set and no longer runny. The edges should be completely set and it should jiggle a bit when you shake the pie dish.
Remove from the oven, let cool, and serve.