In a large sauté pan, sear bacon over medium-high heat until it begins to crisp. Add onions and ground beef, reduce heat and simmer until no pink remains in beef. Remove from stovetop, drain off excess fat and set beef aside.
In a large pitcher combine eggs, sour cream, half & half, milk, salt and pepper. Use a hand immersion blender or whisk to blend until smooth. Set aside. Cut crust from bread and cut into 1/2 inch cubes. Preheat oven to 350°F.
Butter a 9 x 13-inch casserole pan to begin assembly. Layer half of bread cubes followed by beef mixture, chives, spinach, second half of bread and cheese. Pour egg into casserole.
Bake 30 minutes, turn and bake an additional 20-30 minutes until casserole has completely risen in the middle and has a golden crust. Remove from oven. Allow to cool 15 minutes before slicing and serving.