Suggested Beverage Pairing:
Chateau Ste. Michelle Riesling
Place steamer basket into a large pot with a couple inches of water; add the whole sweet potatoes and cover pot with foil. Steam for 30 minutes over medium heat until fully cooked. Place potatoes into refrigerator to cool. Once chilled, peel and cut the sweet potatoes into ½ inch thick rounds (cut into half moons, if necessary, to fit into baking dish) and set aside.
Heat a sauce pan with olive oil and 2 tbsp. butter over medium heat; add the onions and garlic and cook until golden brown. Add the whipping cream, chicken base or bouillon, salt and pepper and bring to a simmer. Cook for 12 to 14 minutes or until reduced by half. Season to taste with salt and pepper; cool to room temperature.
Preheat oven to 375°F. Butter a deep, 1 to 1½ quart casserole dish or cast iron skillet, with 1 to 2 tbsp. butter. Add 2 ounces of cream mixture, then a layer of sweet potato followed by a layer of Gruyère, then Parmesan cheese. Repeat twice, ending with a layer of Gruyère cheese covering the sweet potato-cream mixture. Top with Parmesan cheese and a pinch of black pepper.
Bake for 18 to 20 minutes or until cheese starts to brown. Remove from oven and let sit for 5 minutes before serving, garnish with sliced green onion and enjoy!