You'll Need:9-inch x 9-inch baking dish, mandoline (really helpful and saves a ton of time), nonstick cooking spray, large baking sheet
Preheat oven to 350 degrees.
Spray baking dish with nonstick spray. Place baking dish on baking sheet.
Stir together heavy cream and salt in large bowl. Using mandoline, slice yams into bowl, coating slices with cream mixture as you work.
Layer 1/3 of sweet potatoes in baking dish. Sprinkle with half of cheese. Repeat with another 1/3 of sweet potatoes and remainder of cheese. Top with remaining sweet potatoes. Pour any remaining yam-cream mixture over top.
Bake uncovered on center rack of oven 30 min. Remove from oven.
Press fork or spatula, carefully, against top layer to allow liquid to flow over "crust." Replace pan in oven; bake 30 min. Remove pan; press top layer to allow liquid to flow over top. Replace pan in oven; bake 40-45 min or until sweet potatoes are tender. Remove from oven; let rest 15 min.