Add butter and oil to a large saucepan over medium-high heat. Once the butter foams, add the onion and garlic and saute until soft, about 3 to 4 minutes. Season with salt and pepper, to taste. Add the corn and cook for 1 more minute.
Add the sweet potatoes, chicken broth, apple juice, 2 teaspoons seasoning, white pepper, remaining 1 1/2 teaspoons of salt, allspice, and cayenne. Bring to a boil, then reduce the heat and simmer until the potatoes are cooked through and fork tender, about 20 minutes.
Use an immersion blender and puree the bisque until smooth. Taste for seasoning, then stir in the heavy cream. Rewarm over low heat. Transfer to serving bowls and garnish with Sage and Savory Croutons.
Sage and Savory Croutons:
Heat olive oil in saucepan over low heat. Carefully add the bread to the saucepan and toss until browned. While browning, sprinkle with poultry seasoning. Transfer to a bowl and set aside.