For cashew cream: Soak cashews in 2 cups water at least 4 hours. Drain, reserving liquid. Transfer cashews to a blender. Blend, adding reserved liquid 2 tablespoons at a time until the consistency of buttercream frosting, 6 to 8 tablespoons liquid total.

Preheat oven to 400°F. Line a baking pan with parchment paper. Add mushrooms and 2 Tbsp. oil. Sprinkle with salt and black pepper; toss to coat. Bake 20 to 25 minutes or until mushrooms are crisp and pan is dry, stirring every 5 minutes. Stir in garlic and oregano; season to taste. Reduce oven to 350°F.

For tomato sauce: In a large pot heat remaining 2 Tbsp. oil over medium. Add onion; sprinkle with salt and pepper. Cook 5 to 8 minutes or until onion softens and starts to turn golden, stirring occasionally. Add cinnamon and nutmeg. Cook and stir 30 seconds or until fragrant. Add tomatoes. Bring to boiling; reduce heat. Simmer 30 to 35 minutes or until thickened, stirring occasionally; season to taste.

For mashed potatoes: Place potatoes in a large pot with enough cold water to cover by 1 inch. Add a generous pinch of salt. Bring to boiling. Keep water bubbling steadily until potatoes are fork-tender, 10 to 25 minutes depending on size of pieces. Drain, reserving some liquid.

Using a potato masher, mash sweet potatoes in pot. Add two-thirds of the cashew cream and about 1/4 cup reserved cooking liquid; sprinkle with salt and pepper. If potatoes are too thick, add cooking liquid 1 tablespoon at a time until creamy. Season to taste.

Spread tomato sauce in a 2-quart baking dish. Add mushrooms in an even layer. Carefully spread mashed potatoes on top to completely cover. Spread on remaining cashew cream. Place a foil-lined baking sheet on oven rack below dish. Bake 45 to 50 minutes or until sauce is bubbling and top is browned. Let stand a few minutes before serving. If desired, sprinkle with additional oregano. Serves 4 to 6.