Cook the frozen chopped spinach according to package directions, then drain thoroughly and squeeze out any excess water.
Fry 4 slices of bacon until crispy and break into small pieces.
In a large mixing bowl, combine drained spinach, 1 cup milk, 4 beaten eggs, 1/4 cup chopped onion, bacon pieces, 1/2 teaspoon salt, 1/4 teaspoon paprika, 1/2 teaspoon dry mustard, and 1 cup shredded cheddar cheese. Mix until well combined.
Pour the mixture into the unbaked 9-inch pie crust and spread evenly.
Sprinkle the remaining 1/4 cup cheddar cheese over the top.
Bake in a preheated 400°F oven for 40 minutes, until the center is set and the top is golden brown.
Allow the quiche to rest for 10 minutes before slicing to firm up the custard.