In a large saucepan on medium-high heat, add the ground sausage. Allow it to sear for 3 minutes undisturbed. Then break apart the sausage and cook until browned through. Set the sausage aside leaving behind the oil.
In the same pan, lower the heat to medium and add the onions. Saute for 8 minutes stirring occasionally. Season with salt and pepper. Add the garlic and saute for an additional minute or 2. Set the onions aside.
Add a tablespoon of olive oil to the pan. Add the spinach and saute and wilt for 1-2 minutes. Remove from the pan and use a paper towel to drain the spinach of liquid once the spinach cools.
Preheat the oven to 375℉. Butter the pan then place parchment paper inside of it. Place your pie crust inside the pan. Place another piece of parchment paper over the pie crust. Add your pie weights to the parchment paper. Bake in the oven for 15 minutes. Remove the crust from the oven. Remove the pie weights and brush the crust with an egg wash. Use a fork to make holes in the bottom of the crust. Lower the heat to 350℉ and bake for 5 more minutes.
Combine the eggs and salt in a bowl and whisk together for 2-3 minutes. Then add the heavy cream and milk and whisk for 3-4 minutes.
Once the cooked ingredients cool, whisk sausage, onions, spinach, and cheese into the eggs.
Remove the crust from the oven and pour your egg mixture into the pie crust. Place in the oven and bake for 40-50 minutes.
Notes
Pre-baking your crust is a key step that many recipes will skip. When you pre-bake the crust and use an egg wash, you create a barrier between the crust and the egg mixture. This will lead to a crunchy crust instead of a soggy one.
Drain as much of the oil from the sausage as you can. You don't want all of that fat in the eggs.
When you wilt the spinach, use paper towels to squeeze as much of the liquid out of the spinach as you can. More liquid will make a soggy quiche with poor texture.