Prepare cauliflower rice – If starting with whole cauliflower, cut into florets and pulse in a food processor until rice-sized.
Sauté onion (and garlic if using) – Heat 1 tbsp avocado oil in a large skillet over medium heat. Add onion and cook until soft, about 4 minutes. Stir in garlic for the last 30 seconds.
Add mushrooms – Stir in shiitake and oyster mushrooms. Cook 5 minutes until they begin to soften and release liquid.
Add cauliflower rice – Add remaining oil and stir in cauliflower. Cook 5 minutes, allowing moisture to steam off.
Add coconut milk – Pour in coconut milk, stir well, and cover. Cook 6 to 10 minutes, stirring occasionally, until tender and creamy.
Add arugula (optional) – Stir in arugula for the last 1 to 2 minutes, until wilted.
Finish and serve – Season with sea salt, sprinkle with parsley or thyme, and enjoy hot.