Scalloped Sweet Potatoes with Sage Cream Sauce

Name: Scalloped Sweet Potatoes with Sage Cream Sauce
Ingredients: One medium to large yam or sweet potato (yams add a beautiful bright orange colour to this dish), sliced thin; 2-3 large russet potatoes, sliced thin; 1/2 a large onion, finely diced; 3 cloves of garlic, minced; 1/4 cup butter, plus extra for greasing baking dish; 2 cups whipping cream (or substitute heavy cream); 3 cups (about 300g) of Gruyère cheese, shredded (if you can’t find Gruyère you can substitute Swiss or Emmental); One bunch of fresh sage or 1/4 cup of dried sage, chopped fine; 1/4 tsp. sea salt, plus more to taste; Fresh cracked pepper to taste