The most amazing tomato soup recipe I can't find

It's probably tomato puree with some butter along with a masala. A masala (spice mix) can easily have 10+ ingredients and many Indian families have their own traditional recipes with various ratios of the spices. If you're near an Indian grocery, they may have some pre-mixed masalas of various types that you can play with to approximate the taste you remember. There are likely many online spice vendors who could be a source if you're not near an Indian grocery. Unfortunately, although there are some more standardized masalas (like garam masala), many masalas are more like a fingerprint and can be challenging to duplicate. That said, with your memories being 10 years old, a garam masala and a general mixed masala together could get you very close to what you remember.

Maybe it was http://myfancypantry.com/2013/05/04/tamatar-shorba-indian-spiced-tomato-soup/?

I wonder if it might be a rasam (Indian soup) and a tomato-based one is called https://www.google.com.au/#q=Thakkali%20Rasam%20recipe. Sometimes it can be made sour with tamarind, but often it's a plain tomato and spice recipe. Try looking at some of the recipes and see if something sounds familiar?

My mom puts cumin and coriander seeds in a can of Campbell's and it tastes amazing. Perhaps it was something similar?

I do not have a specific recipe but an easy place to start would be http://ask.metafilter.com/253698/The-most-amazing-tomato-soup-recipe-I-cant-find#3685305, using ghee/clarified butter regular butter (or oil). However you make it, dry toast the spices in the pan first over med-hi heat, add the ghee, mix quickly to a paste, then add the pureed strained tomatoes.

I can tell you that Indian tomato varieties are not hybridized for color, robustness and longer shelf life, are not stored in the refrigerator (they lose their flavor below 50 degrees F) and extremely delicious by themselves unlike what you can purchase in the US. Better tomatoes equals better soup.

If it's the same Indian tomato soup I love and crave, you'll have success searching for recipes with "rasam" in the name. They often involve tamarind, which gives it a delicious tangyness. http://www.seriouseats.com/recipes/2010/08/rasam-indian-tomato-soup-recipe.html, while minus the tamarind (I imagine that's what the optional brown sugar is supposed to account for) and not directly from an Indian source, comes very close to what I get from the Indian places in the city.

Was it a vegetarian soup? What ethnic group of India was your friend from? That might help narrow it down.

Mulligatawny? It's something like Indian minestrone, which I always associate with tomatoes.