Cook 8 strips of bacon in a large skillet over medium heat until crispy, about 8-10 minutes. Transfer to paper towel to drain.

In the same skillet with reserved bacon fat, add 1 diced onion and sauté over medium heat for 5 minutes. Stir in 2 minced garlic cloves and 5 cups of spinach. Cook until spinach wilts, about 2-3 minutes.

In a mixing bowl, whisk together 8 eggs, 2 cups of half and half, and add salt and pepper to taste. Mix in 2 tablespoons of chives.

Grease a 9x13 inch baking dish. Layer 5 cups of bread cubes, then the spinach mixture, crumbled bacon, and half of the shredded cheese. Pour the egg mixture over top and sprinkle with remaining cheese.

Cover with plastic wrap and refrigerate overnight or at least 6 hours.

Preheat oven to 350°F (175°C). Allow casserole to come to room temperature for about 20 minutes. Bake uncovered for 35-40 minutes.

Once done, let cool for 5-10 minutes before cutting into squares and serving.