In food a processor, chop the mushrooms and onion finely. In a hot pan, heat oil and saute the chopped vegetables.
To cook the spaghetti squash, cut in two and empty take out the seeds. Place on a cookie sheet and bake at 350F for 35-50 minutes, or for 10-15 minutes or in the microwave. Take out the pulp with a fork.
In another pot, cook the turnip, potato, butternut squash and celeriac in salted water. When tender, puree and add margarine.
Preheat oven to 350 F.
Into 8 small individual ovenproof containers (or a big one), separate the mushroom mix. Divide the cooked spaghetti squash on the mushrooms. Spread mashed vegetables on top.