Preheat oven to 375F and place pre-made pie crust into a 9-inch tart dish, set aside.
In a large frying pan fry bacon for 8 minutes.
Add spinach and cook for 1 minute. Remove spinach and bacon to a paper towel lined bowl.
In same skillet, caramelize the onions in the butter, 10-15 minutes. Cover skillet. Season with 1/2 tsp salt and thyme. Stir frequently. Remove from heat, allow to cool for 5 minutes.
Once cool chop bacon into small pieces.
In a large bowl whisk together eggs, heavy cream, remaining salt, and pepper.
Stir bacon mixture and onions into eggs. Combine.
Pour into pie crust pour.
Bake for 15 minutes until set and golden brown. Turn oven down to 325. Continue to bake 20 minutes.