- Pre-heat the oven to 400°Fahrenheit. Grease a 9x9 baking dish with butter or oil of choice and set aside.
- In a large saucepan, heat the butter over medium-high heat.
- Add onion and garlic and cook until onion is translucent about 5 minutes.
- Add spices and cook for one minute until fragrant.
- Add arrowroot flour and stir for one minute until well combined.
- Pour in cashew milk. Mix well. Bring sauce to a boil. Once at a boil, lower heat and let simmer until thick enough to coat the back of a spoon. Add in 1/2 cup of parmesan cheese (if using) and whisk until cheese has melted, about 30 seconds. **Make sure the mixture has thickened a good amount before adding spinach.
- In batches, add spinach (and kale if using) to the sauce until the greens start to wilt. Taste and adjust seasoning as needed. Take off heat.
- Pour spinach/kale mixture into a greased 9x9 casserole dish and top with remaining parmesan cheese. Bake for 20 minutes or until bubbly and the cheese on the top is a golden brown.
- Serve warm!