1. Preheat oven to 400ºF. Line a baking sheet with foil and place in oven. Combine mozzarella, ricotta, spinach and salt in a bowl.
2. Using a large serrated knife, carefully slice off the very top of each bagel and discard. Scoop out bagels' doughy interiors, leaving crust intact. Discard interiors.
3. Spread 2 Tbsp. tomato sauce inside bottom of each bagel. Divide cheese mixture among bagels, spreading evenly.
4. Place bagels on preheated baking sheet and bake until cheese is bubbling and bagels are crisp and browned around cut edges, about 15 minutes.