Heat a large skillet (I prefer cast iron because it gets super hot) over medium-high heat. Add 1 tablespoon of the ghee. When the ghee is hot and shimmery, add the mushrooms and a pinch of salt. Cook, stirring a few times, for about 5 minutes. You want most of the moisture to cook off and the mushrooms to get a little caramelized on the edges. Remove the mushrooms and set aside in a bowl.
Reduce the heat to medium. Add another 1 tablespoon of ghee to the same skillet. Immediately add the garlic and bay leaf, cooking and stirring for about 30 seconds, until the garlic is fragrant. Add the celery and onion. Cook and stir often until the veggies are tender and translucent but not browned, about 8 minutes.
While this is cooking, measure out 1 cup of coconut milk. Remove 2 tablespoons of that into a small bowl. Add 1 tablespoon of arrowroot flour. Stir with a spoon until the arrowroot is dissolved. Set the slurry aside.
Stir in the arrowroot slurry. It will thicken almost immediately.
Add the frozen peas. Remove the bay leaf.
Adjust the seasoning with salt and pepper. Serve over roasted spaghetti squash.
* For 100% dairy-free, use coconut or avocado oil instead of ghee.
**If you don't have arrowroot, you can use tapioca flour just the same.