Thoroughly clean the mushrooms and slice them into thick slices (approximately 1/2-⅜ inch thick). Cut the onion into 3/8-½ inch thick half-moon slices.
Preheat the Blackstone grill over medium high heat (approximately 450 degrees F). Then spread the avocado oil and the butter onto the grill. Once the butter is melted, add the sliced mushrooms and onions to the grill. Season them with the salt and pepper.
Let the mushrooms and onions cook on the griddle for 2-3 without tossing them. Then gently toss the mushrooms and cook for 2-3 more minutes until the mushrooms are browned on both sides. Then continue to toss the mushrooms and onions on the grill until they are the tenderness that you prefer (2-3 more minutes).
Remove from the grill and serve as it or over steak. Enjoy!
RECIPE NOTES:
These mushrooms and onions are best served fresh, but you can refrigerate any leftover mushrooms in an airtight container for up to 3 days.