Preheat the oven to 350 degrees.
Place all the ingredients for the breadcrumb topping in a food processor or bowl and combine to mix well. Set aside.
Place the dry mushrooms in a bowl and pour 1 1/2 cups of hot water to reconstitute. Leave alone for at least 10 minutes while you prep the other ingredients.
Place the raw cashews in a blender along with the remaining 1 cup water, onion powder, nutritional yeast, dry rosemary and fried onions. Process into a very smooth paste. If you're not using a high-speed blender, you will need to soak the cashews first for 30 minutes before adding them to the blender.
In a 12-inch cast-iron skillet, heat the oil. Add the leeks and saute over medium-high heat about 5 minutes or until they are soft and beginning to brown. If you don't have a stove-to-oven skillet, use a skillet for the first seven steps and then pour the casserole ingredients into an oven-safe baking dish before adding the breadcrumb topping.
Add the garlic and fresh herbs and continue to saute. Add the mushrooms, both the frozen and the reconstituted dry mushrooms. Reserve the stock from the mushrooms.
Let the mushrooms continue to cook for another five minutes, stirring frequently. If you replace the frozen mushrooms with the fresh, make sure you saute the mushrooms until most of the moisture has evaporated from the skillet. This will add some more cooking time. Add the rice and cook another couple of minutes, stirring well to mix.
Turn off the heat. Pour in the cashew cream sauce and the mushroom stock. Mix well and check seasoning. Add more salt and ground black pepper if needed.
If you need to transfer your ingredients to a casserole dish, do it at this point. Spread the breadcrumb topping evenly over the top of the casserole. Cover the casserole with aluminum foil.
Place the skillet or casserole dish into the preheated oven. Bake 45 minutes, then remove the foil and continue baking another 15 minutes until the breadcrumb topping is golden. If you skip the topping, keep the foil on for the full hour of baking so your casserole won't dry out.
Remove the casserole from the oven, and let it stand five minutes before serving.