What's the most common type of ethnic food on the Eastside?
By that I mean what's the highest concentration of ethnic food available on the Eastside - Indian? Mexican? Asian? - and in what community(ies)?
The Eastside in general doesn't have a great deal of concentrated ethnic enclaves as compared to what you might find in New York, Los Angeles, Chicago, and other larger cities with bigger immigrant populations. In general I would say the Eastside gems tend to be spread out rather than clustered together on a few blocks, which is different from the Ethiopian strips in the Central District or the Mexican/Central American neighborhoods in Burien and White center, for example. Of course, there could be hot spots I have not been fortunate enough to know about yet.
Redmond (also Renton and Kent) seems to have a good number of Indian restaurants and groceries. The Japanese supermarket Uwajimaya is there, and while it is not as good as the Seattle branch, it attracts a diverse clientele.
The crosscroads area of Bellevue has a number of ethnic groups represented, and in and near to the Crossroads mall complex you will find taquerias, mexican groceries, Russian delis, and a place serving Dosai and other Indian vegetarian items. In Lake Hills there is a "European" deli, an Iranian grocery, and a Korean supermarket.
I have also seen several ethnic groceries, including a brazillian market, in the Totem Lake area of Kirkland.
Speaking in raw census percentages, the largest minority group on the Eastside is still "Asian". I would say that this also makes for the most readily available ethnic food. This means Thai, Sichuanese, Japanese, Cantonese, Vietnamese, Korean, Taiwanese, and other more Americanized, generic or pan-Asian takes on the many Asian cuisines (e.g. Teriyaki).
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Bellevue seems to have an unusual number of Indian places. The best used to be Mayuri, but alas I think the ownership has changed. We went in about 3 months ago and they sat us at a dirty table with the water half gone out of one glass and the ice melted in both. When I brought it up, they wanted to argue that the table had been turned. We walked and haven't been back since.