Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish. Tear bagels into bite-sized pieces and spread them in the dish.

Cook bacon in a large skillet until crispy, about 5-7 minutes. Remove, chop, and set aside. In the same skillet, sauté onion and spinach in bacon fat until softened, about 3-4 minutes. Combine with bacon and spread over bagels.

In a mixing bowl, whisk together eggs, milk, and seasonings. Pour the egg mixture over the bagel and vegetable layers, letting it seep through.

Cut cream cheese into small chunks and distribute over the casserole. Mix gently with a fork.

Cover with aluminum foil and bake for 45-55 minutes, until set and golden brown. Let cool for 10 minutes before slicing.