Preheat your oven to 375°F (190°C) and reheat the spaghetti sauce, if necessary.
Place the cauliflower florets, water and salt in a large stock pot, cover and cook over high heat for about 2 minutes, until the cauliflower is just slightly softened. Drain the water well and return it to the stock pot. Place the stock pot with the cauliflower in it back over the heat source and cook over medium heat this time for about a minute, just to dry the cauliflower out.
Add the hot spaghetti sauce and cubed mozzarella cheese to the cauliflower and mix delicately until all the ingredients are well combined.
Transfer to a large baking dish and top with the grated mozzarella and sharp cheddar cheeses.
Bake for 30 minutes or until the cheese is fully melted and starts to bubble and turn nicely golden brown.
Remove from the oven and allow to rest for about 10 minutes before serving.