According to the CIA Factbook, the various ethnic groups in Russia are Russian 77.7%, Tatar 3.7%, Ukrainian 1.4%, Bashkir 1.1%, and all other groups less than 1%. This is pretty close to monoethnic.
Moving west, Ukraine is almost entirely Ukrainian and Russian; Belarus is Belarusian and Russian; Romania is Romanian and Hungarian; Poland is almost entirely Polish; Germany is mostly German; France doesn’t even bother to distinguish its ethnic groups; nor does Spain; and the UK just defines itself as “white”, without intentionally obfuscated ethnic groups.
The UK does this primarily, not out of liberal sentiment, but because it is a conglomerate of four distinct nationalities (along with various other subgroups from throughout the former empire).