Poke spaghetti squash all over with a knife to allow steam to escape while cooking. Microwave on high for 1-2 minutes.
Remove spaghetti squash from the microwave and cut in half. Scoop out seeds.
Place each half of the spaghetti squash face down in a microwave-safe platter with about 1 inch of water. Microwave on high for 5 - 10 minutes or until the outer skin is easily pierced with a fork.
Remove spaghetti squash from microwave, turn each half over so the strands are facing up and use a fork to remove strands to a bowl. Measure out 2 cups of strands and reserve the remainder for another use.
Heat oil in large pot over medium heat. Note the following steps can be started while the spaghetti squash is cooking.
Add onions, celery and carrots and saute until tender, about 5 minutes.
Add garlic and saute for 1 minute more.
Add remaining ingredients to the pot, except for the spaghetti squash. Note if the spaghetti squash was previously cooked and refrigerated then go ahead add it now too.
Bring soup to a boil, reduce heat to low and simmer for about 10 minutes.
Stir in spaghetti squash and enjoy.