1. To prepare the squash: In a shallow, non-reactive baking dish, roast the squash at 375F for 30-45 minutes or until squash is soft when punctured with a fork.
2. When cool enough to handle, cut squash in half longwise. Scoop out seeds and discard. Gently scoop out “spaghetti” flesh and set aside.
3. To make the porcini mushroom broth: Peel and roughly chop celery, carrot, and one third of the onion.
4. Add chopped veggies to a saucepan with the water, bay leaf, and dried porcini mushrooms. Bring to a boil, then lower to a simmer and cook for 20-30 minutes; strain.
5. Remove the rehydrated mushrooms, chop and set aside. Discard the celery, carrot, and onion from the broth.
6. Dice the remaining onion.
7. In a large skillet, heat half the olive oil over medium-high heat. Add diced onion to the hot oil; cook until translucent.
8. Lower the heat to medium, season the onions with a generous pinch of sea salt and a few grinds of black pepper, and continue stirring until caramelized, about 5 minutes.
9. Remove the onion to a small bowl. Return the pan to the heat, add the remaining olive oil, and turn the heat to high. Add all the mushrooms, stirring well to coat with oil.
10. Keep the heat high and sauté the mushrooms until they brown lightly. If juices accumulate in the pan, pour them off and reserve.
11. Add garlic, thyme, sage, and pepper flakes; stir well. Reduce the heat to medium, add the caramelized onions and the tomato paste, and stir to combine.
12. Cook for another 2 minutes, stirring.
13. Sprinkle the arrowroot over the mixture and combine. Ladle in 1 cup (per 4 servings) of the hot mushroom broth, stirring well as the mixture thickens.
14. Add another cup of hot broth and let the ragout cook for another 5 minutes. If it’s too thin, cook it a bit longer; if too thick, add a bit more broth.
15. Adjust with sea salt and freshly ground pepper to taste.
16. Mince parsley.
17. Add spaghetti squash to the skillet along with avocado oil and about 1/4 cup (per 4 servings) parsley (reserve some parsley for garnish); combine well and heat through.
18. Transfer a small amount of spaghetti squash to separate serving dishes (you want to err on the side of more ragout).
19. Top generously with mushroom ragout, and serve hot topped with a sprinkle of parsley.