Preheat the oven to 400º Fahrenheit

Heat the olive oil in a large sauté pan over medium heat. Add the mushrooms and diced onion and stir for a few minutes. Add the broccoli florets (make sure they're in bite-sized pieces and not too large) and stir for another 2 minutes. Next, add your cauliflower rice and cook for another 2 minutes.

Place the shredded chicken in a 9x13-inch casserole dish, covering the bottom. Add the sautéed mushrooms, onions, broccoli and cauliflower rice in a flat layer on top.

Next, make your sauce.

Add the onion and 1 cup of the broth to a large pan over medium to medium-high heat. Cook for about 8 minutes until the onion is very tender. After 8 minutes, add the garlic and cook a couple of minutes more, stirring often. All of the broth should have evaporated by now. If not, keep letting it cook until the broth is all gone. You don't want any more liquid once it's done cooking, so the end result is thick and creamy.

Add the cooked onion and garlic to a blender. Add 1/2 cup more of the remaining broth and the remaining ingredients (salt, pepper, cashews, lemon juice, nutritional yeast). Blend on high for a couple of minutes until very creamy and smooth. Add any more broth to reach desired consistency, if necessary. Taste and add any more salt & pepper or lemon, if desired.

Pour on top of the mixture in the casserole dish. Use a spatula if needed to make sure everything is well coated.

Cook the casserole for ~20 minutes. The sauce should be bubbling and slightly reduced and thickened. Garnish with fresh rosemary if desired and serve immediately.