Preheat the oven to 350°F (180°C).
In a food processor or bowl, combine all the ingredients for the breadcrumb topping. Mix well and set aside.
Place the dry mushrooms in a bowl and pour 1 ½ cups of hot water over them to reconstitute. Let them soak for at least 10 minutes while you prepare the other ingredients.
In a blender, combine the raw cashews, 1 cup of water, onion powder, nutritional yeast, dried rosemary, and fried onions. Blend until a very smooth paste forms. If not using a high-speed blender, soak the cashews for 30 minutes before blending.
Heat oil in a 12-inch cast-iron skillet. Sauté the leeks over medium-high heat for about 5 minutes until soft and beginning to brown. If not using a stove-to-oven skillet, use a regular skillet for steps 5-7, then transfer to an oven-safe baking dish before adding the breadcrumb topping.
Add the garlic and fresh herbs to the skillet and continue to sauté. Add the frozen mushrooms and reconstituted dry mushrooms, reserving the soaking liquid. Cook for another 5 minutes, stirring frequently. If fresh mushrooms are used instead of frozen, sauté until most of their moisture has evaporated, which will take longer.
Stir in the rice and cook for a couple more minutes, mixing well. Turn off the heat. Pour in the cashew cream sauce and reserved mushroom stock. Mix thoroughly and adjust the seasoning with salt and black pepper to taste.
Transfer the mixture to a casserole dish. Spread the prepared breadcrumb topping evenly over the casserole and cover it with aluminum foil.
Place the skillet or casserole in the preheated oven. Bake for 45 minutes, then remove the foil and bake for another 15 minutes until the breadcrumb topping is golden brown. If omitting the topping, keep the casserole covered for the full hour of baking to prevent drying out.
Remove the casserole from the oven and let it stand for 5 minutes before serving.