Preheat oven to 350 degrees F (176 C).
Roll out the dough and place in a 9-inch pie plate, or if using a ready-made crust, defrost according to package directions.
Heat the olive oil in a large skillet over medium-high heat. Once hot, add the diced onion and let sauté for 3 minutes. Then add the minced garlic and diced bell pepper and cook for 3 minutes, stirring occasionally.
Then add in the breakfast sausage, and break it up with a spoon or spatula. Let sauté for 3 to 5 minutes until it browns. Add the spinach and cook just till the spinach has wilted.
Spread the sausage onion mixture across the base of the pie dough in an even layer.
Crack the eggs into a mixing bowl. Whisk till the yolks break. Add in the milk, cheese, salt, and pepper. Whisk till combined.
Pour the egg mixture on top of the sausage mixture.
Bake for 45 to 50 minutes or until the center of the quiche has set and the crust is golden brown. Allow the quiche to cool for 5 to 10 minutes before slicing and serving.