Making perfect sense. I suspected there had to be more of a difference than just substituting shrimp for the chicken. The chicken tikka masala that I've had in the past (or stolen from my wife's plate when she used the restroom) is a spicier dish in that the spices seem to take precedence over the flavor of the chicken. The tandoori shrimp masala tends to have more of a balance between the natural flavor of the shrimp and the spices used. The mixture does wind up a pinkish-orange but again, a lighter hue than that of the chicken tikka masala. I definitely think you're on the right track with the onions and other ingredients creating a lot of the body of the sauce through emulsification. I do think there is at least some cooking of the mixture with the shrimp as the flavors just seem to work in such a way that I'm not sure you'd get if you made them separately. I could be completely wrong there but it just really tastes like that to me. Might have to go on another fact finding mission to my local Indian resturaunt.
Just 'cause I like to make a lot of this stuff at home doesn't mean I can't do my part make sure they stay in business!
As for availability here in the great heartland, we've got all kinds of ethnic grocery stores in metro Kansas City. We've got quite an Asian population, including Indian, so finding the required spices shouldn't be too difficult. Some possible substitutions that might be found at a chain grocery could come in handy as well, if only to road test both versions for people who might not be as lucky in other areas. I've frankly been amazed at the diversity of specialty / ethnic grocers we have in the area. I mean, this
is Kansas City, after all, a city where most people think food diversity means you have your choice of rib joints. Which we do. Which are awesome.
I'd love to see an actual recipe if you'd like to write it down. I've not got enough experience with Indian style cooking to competently freelance with it just yet. Not that I'm usually stopped by that fact.