Preheat the oven to 450F and move the rack to the middle. Line a baking sheet with parchment paper and liberally grease it with olive oil. Set aside.
Place the cauliflower florets in the food processor and pulse until it is about the size of rice. It should be finely chopped but not pulverized.
Transfer the cauliflower to a microwave-safe bowl and microwave on high for 8 minutes, until cooked and soft.
Allow the cauliflower rice to cool so that it is easier to handle. Place in a tea towel and squeeze out as much moisture as you can. This is a very important step as the cauliflower rice needs to be dry to form into bread slices.
Place the dry cauliflower in a mixing bowl, add the eggs, mozzarella and salt and mix well.
Divide the mixture into 8 equal sections, and shape each into a square on the parchment lined baking sheet.
Place in the oven and bake for about 16 minutes, until golden. Remove from the oven and allow to cool 10 minutes before removing from the parchment paper.
To make the grilled cheese sandwiches: heat the butter in a pan over medium heat.
Place one slice of the cauliflower bread into the pan, top with cheese, and another slice of bread.
Cook for 2-4 minutes on each side, until the cheese is melted and the bread is golden brown.