Wild Mushroom and Root Vegetable Gratin

Name: Wild Mushroom and Root Vegetable Gratin
Ingredients: Kosher salt, to taste; 1 1/2 lb. (750 g) Yukon Gold potatoes, peeled and sliced 1/4 inch (6 mm) thick; 3 lb. (1.5 kg) rutabagas, peeled and sliced 1/4 inch (6 mm) thick; 1/4 cup (60 ml) extra-virgin olive oil; 2 Tbs. finely chopped fresh rosemary; 2 Tbs. grapeseed oil; 13 1/2 oz. (425 g) wild mushrooms, such as chanterelles, black trumpets and maitakes, cleaned and roughly chopped; 1 lb. (500 g) Blackberry Farm Brebis cheese, goat cheese or fromage blanc, crumbled; Maldon sea salt, to taste