Hazelnut Hasselback Potato Gratin

Name: Hazelnut Hasselback Potato Gratin
Prep time: 25 min
Cook time: 1 h 20 min
Total time: 1 h 45 min
Cuisine: European, French, Mediterranean
Ingredients: 3 tablespoons (42 grams) regular or vegan butter; 3 tablespoons (30 grams) rice flour or all purpose flour; 3 cups plus 2 tablespoons (750 milliliters) hazelnut milk (see Note); 1 garlic clove, finely chopped; of freshly grated nutmeg; 1 sprig of rosemary, leave picked and finely chopped; 1 tablespoon soy sauce or tamari; sea salt and black pepper; 1 leek, white and green parts finely sliced and washed well; extra-virgin olive oil; 3 pounds 5 ounces (1.5 kilograms) potatoes (any variety), scrubbed; 15-20 sage leaves