The phrase "Grattez le Russe, et vous verrez un Tartare" (Scratch a Russian and you'll find a Tatar) wasn't invented yesterday.

"Slavic Russians" are mostly Belarusians (Smolensk Region) or forcibly assimilated Ukrainians (Bryansk, Kursk, Voronezh) or not even assimilated Ukrainians in Kuban (they still speak Ukrainian, but have their own name for it).