This is the Holy Transfiguration of Christ Cathedral in North Denver. This Cathedral is a Cathedral of the Orthodox Church in America, a church which traces its origin to the Russian Orthodox Church after the Russian Revolution. The particular neighborhood this church is in must have been heavily Slavic at one time, as there is a Catholic Church which was built for a largely Polish population all within a couple of blocks, with a third Catholic Church nearby.. Recently I observed a Polish flag, flying with the US flag, on a Catholic school in the neighborhood, but are the area is largely Hispanic today.
There are two bodies that descend from the Russian Orthodox Church in the US today, and I frankly don't quite understand the relationship between the two, but this Cathedral in Denver reflects part of Denver's Russian Orthodox community. The church dates to 1898.