Place the cauliflower, rosemary and garlic in a large pot and cover with chicken stock. Bring to a boil over medium-high heat. Boil until fork-tender, about 10 minutes.
Drain and remove the rosemary sprigs. Put the cauliflower and garlic in a food processor and blend until smooth.
Using the same pot, add the milk and olive oil and warm through, about 5 minutes.
Add puréed cauliflower mixture back into the pot. Stir everything together using a rubber spatula.
Add the mozzarella, parmesan and salt and continue to stir until everything has combined and the cheese is melted. About 5 to 7 minutes. It will start to bubble a bit.
Serve hot.