Preheat the oven to 375 degrees.
Spray a pie dish with cooking spray then place the prepared pie crust in. Crimp the edges.
Heat a nonstick skillet over medium-high heat. Add the sausage, removed from the casings. Cook for 1-2 minutes, making sure to break up the sausage into crumbles (if you are using lean sausage, add some olive oil to the pan).
Add the onion and cook, stirring occasionally, for 2-3 minutes, or until just about cooked through (it will finish cooking in the oven).
Add the minced garlic and cook, stirring constantly, for 1 minute.
Drain on a paper towel if needed. Set aside.
Combine the eggs, milk, sea salt, and freshly cracked pepper, to taste, together in a bowl and whisk until very well combined.
Sprinkle most of the sausage mixture in the bottom of the pie crust followed by most of the spinach, grape tomato halves, and cheese.
Add the well-whisked egg mixture. Sprinkle the remaining sausage mixture, spinach, tomatoes, and cheese on top.
Carefully place into the oven and bake for 40 minutes, or until the top is set and not wiggly and the eggs are cooked through. Let the quiche cool for at least 15 minutes before slicing and serving. Enjoy.