Chop cauliflower into 1 inch florets. Add to a food processor or high-powered blender (if using a blender, add a cup or two of water). Process until finely chopped (drain if using a blender with water).
Place in a microwave safe bowl and cover with plastic wrap. Microwave on high for 10 minutes. Be very careful when removing plastic wrap as the escaping steam can burn you!
Set a tea towel into a sieve and drain cauliflower through the tea towel. Let sit until cool enough to handle and then gather the ends of the tea towel together and squeeze out as much liquid as possible. Preheat oven to 425F.
Return to bowl and add mozzarella, eggs, garlic, salt and pepper. Mix well. Line a large baking sheet with parchment paper and pat cauliflower mixture into a large, flat rectangle, about 12 by 10 inches. Bake until golden brown and becoming crisp on the edges, and firm in the center about 25 minutes.
In a small bowl, whisk together the mustard, mayonnaise and nutmeg. Spread over crust. Arrange ham to cover and then sprinkle with both cheeses.
Bake another 5 to 10 minutes, until cheese is melted. Broil a minute or two for nicely browned topping.