Have ready your pie crust, either home made or store bought. (See headnotes above)
Pre-heat the oven to 375°F (190°C) and place the rack in the middle position.
Place the frozen spinach in a microwave safe bowl, cover with a plate and thaw according to the package instructions.
Fry the bacon in a 10" (26cm) non-stick skillet to your preferred doneness. Using a slotted spoon, scoop out the bacon on to a paper towel lined plate. You should have about 1/4 cup (60ml) of rendered bacon fat remaining in the pan.
Over medium heat fry the onions in the bacon fat for about 5 minutes until they are soft and translucent. They should not take on any color.
Next, add the garlic and cook 30-60 seconds until it becomes fragrant.
Sprinkle the flour over the onions and stir with a rubber spatula to make a roux. Cook, stirring continuously for a minute or two.
Pour in about 1/2 of the milk and stir vigorously to blend in the roux. Bring to a simmer and as the mixture begins to thicken add the rest of the milk. Cook, stirring often for about 5-6 minutes.
Season with 1/2 Tsp of salt, 1/4 Tsp of pepper. Stir in the 1/4 Tsp of nutmeg and optional 1/2 cup of dill.
When the mixture returns to a simmer, add the spinach and stir thoroughly to combine.
Gently stir in the crumbled feta cheese. Don't over mix; you want some little pockets of cheese in the mixture.
Taste for seasoning and adjust to your liking.
Pour half the spinach mixture into a 2Qt (2L) oven safe dish and sprinkle the reserved bacon bits over.
Spread the remaining spinach mixture over the bacon.
Cover with the pie crust. Bake for 35-40 minutes until the filling is bubbling and the crust is golden brown.
Rest the pie 15 minutes before serving.