• Preheat oven to 375°F . Prepare casserole baking dish by rubbing about 1/2 tbsp of butter along bottom and sides.
• Cut bagels into 2" cubes, place on baking sheet and toast in oven for 10 minutes. Then remove from oven and let cool. Cube tomatoes and set aside.
• Cube tomatoes and set aside. Using a clean kitchen towel, wring and squeeze spinach over sink to remove excess moisture, set spinach aside.
• Melt butter in skillet over medium-low heat. Add sliced onions and cook slowly for 10-15. Stir periodically to prevent burning. You want a golden brown color.
• In medium sized mixing bowl, whisk together: eggs, half & half, milk, dijon mustard, salt, pepper, cayenne pepper, and grated parmesan cheese.
• In large mixing bowl, combine onions, soften cream cheese and spinach. Stir to just combine. Its' okay to have clumps of cream cheese (in fact, its preferred!) Add cubed bagels and tomatoes to spinach mixture. Toss to combine, then transfer to prepare casserole dish.
• Press down lightly so filling is not heaping over. Pour in egg mixture. Top with shredded mozzarella and everything bagel season. Cover and let chill in fridge, for 2 hours to overnight.

To Bake:

• When ready to bake, remove from fridge and let sit out at room temp for 30 minutes before baking.
• Meanwhile, preheat oven to 350°.

  Bake covered for 30 minutes, remove foil and bake an additional 20-30 minutes. You will know it's done when the center doesn't jiggle when shaken.Remove from oven at let it sit for at least 5 minutes before serving.