Potato and Celery Root Gratin

Name: Potato and Celery Root Gratin
Ingredients: 3 tablespoons butter (plus more for greasing); 3 tablespoons all-purpose/plain flour; 1 garlic clove (peeled and finely chopped); 1 ½ teaspoons fresh rosemary (chopped); 1 ½ cups (354 ml) whole milk, room temperature; ½ teaspoon salt; 1 ½ pounds (680 grams) potatoes, peeled and thinly sliced (*see note); ½ pound (226 grams) celery root/celeriac, peeled and thinly sliced; 1 cup (86 grams) white cheddar cheese, grated; ½ cup (45 grams) Parmesan cheese, grated