Preheat oven to 375°F (190°C). Spray one 12-well muffin tin (or two 6-well muffin tins) with cooking oil spray, and set aside.
Put thawed spinach into a mesh strainer, and squeeze out as much excess water as possible. Set spinach aside.
In a large mixing bowl, coat cubed bread with olive oil. Add spinach, cheese, green onions, garlic powder, salt, pepper and bacon. Toss the bread mixture until well combined. Divide the mixture evenly among the 12 muffin-tin wells.
In the same bowl, beat the eggs and stir in the milk. Pour the mixture over the bread in the muffin tin, trying your best to evenly divide the egg-and-milk mixture into each muffin well.
Bake the casseroles for 30–40 minutes, or until the bread on top is golden and crispy and the centers are set. Test by poking a skewer into one of the casseroles, and see if it comes out clean.