In a medium, microwave-safe bowl, combine the onion, garlic, and water. Season with salt and pepper to taste and cover tightly with plastic wrap or covering. Microwave for about 10 minutes.
Pour the mixture into a blender or food processor and blend until smooth. Season with salt and pepper, to taste, if needed.
Heat a large skillet under medium high heat and add the onion-garlic puree, cabbage, and tomatoes.
Cook pasta in a large pot of boiling water, according to package directions for al dente. Drain and set aside.
In a small bowl, whisk the heavy cream with the cornstarch until completely dissolved.
Add the cornstarch mixture to the onion-garlic in the skillet. Raise heat to high and bring to a boil, whisking constantly. Reduce heat to low and simmer for about 1 minute, or until slightly thickened.
Add the Parmesan cheese and nutmeg to the sauce. Whisk until cheese is melted. Season with salt and pepper, to taste.
Add noodles to mixture and toss well. Serve right away!