What vegetable for sauteed zucchini could I use instead of tomatoes

Here is a good list of choices for substitutions: http://foodallergies.about.com/od/cooking/p/cookingwoonions.htm.

The thing I found interesting though, is what it says about Garlic and Onion powder:

Garlic powder and garlic salt are made from dehydrated, finely ground garlic. Onion powder and salt, likewise, are made from onions. These products are not suitable for anyone with a true allergy to either of these foods. However, some people who do not have true allergies to these foods and avoid garlic and onions due to gastrointestinal upset find that these products do not aggravate their symptoms, especially if used in small amounts.

So the question becomes, is it an actual allergy? You may want to check with the doctor on that one.

Here's their list of substitutions:

• Fennel has a licorice-like taste but onion-like texture. Try it with chicken or fish.

• Celery is among the most common aromatics.

• Bell peppers are often used in Cajun cooking. Green peppers and celery are a good base for rice dishes or savory stews.

• Carrots are used as an aromatic in French cooking in combination with celery.

• Celeriac, or celery root, is the knobby root of one variety of celery. Peeled and diced, it can be used as an aromatic in sauces or stews.

• Peppercorns: white, pink, or Szechuan pepper can add different flavors to your cooking.

• Cumin's distinctive taste that may work well in some recipes, especially where garlic is used raw.

• Horseradish, freshly grated, can add some of the pungent notes you might otherwise lack.

• Ginger and galangal have distinctive flavors but may be useful in stir-fries as aromatics.

One of the first things I learned in Indian cooking is that the combination of tomatoes, onions and ginger is self-thickening. As time went by, I realised that the thickening effect is far more noticable with old varieties of tomatoes - "beef" tomatoes and a lot of the modern varieties are difficult to thicken unless partially fried first.

Despite the absence of ginger, I suspect that the thickening is purely a natural action between the tomatoes and the onion, and that the tomatoes used were some particularly nice old variety.