In a large skillet cook bacon until crispy and brown. Set bacon aside on paper towels to drain. In the same pan with a little of the bacon grease left, saute the chopped onion until well softened and beginning to caramelize. In a large bowl mix together the eggs, milk and 1/2 t salt and pepper each with a whisk. Add cheese and spinach and mix. Add bread cubes and toss until well coated and all ingredients are distributed. Spray baking dish with non-stick spray. Pour out into large flat casserole or baking dish 9x15-inch (3 quart at least). Cover with foil and refrigerate overnight. Preheat oven to 350°F. Bake the casserole, uncovered, until set and lightly browned on top about 30 to 40 minutes. Remove from oven and cut into squares. Notes: You can substitute any good strong aged cheddar. You can also use sourdough bread or any other bread if you can't find the Kings Hawaiian rolls.