where they serve Kozel in Warsaw
The best Kozel I've had in Warsaw is at Ceska (ul. Chmielna 35, behind Galeria Centrum) but frankly it's a waste to drink Kozel there as that place has a Pilsner Urquell tankowy system and thus serves up Mliko, Na dvakrat and Hladinka. Sadly the service is at best somewhat lacking. The only other tankowy system in Warsaw is at Aioli on pl Konstytucji; but that place is best avoided, the Pilsner I had there was undrinkable.
But you'll do far better at Warsaw's pubs that specialise in Czech beer: Czeska Baszta (hard to find, it's hidden under a bridge and smells, but also has the best beer), Czeska Piviarnia (a bit out in the sticks for me but worth the trip if you like Czech beer), Czeska Piviarnia Hospoda (not been, way too far from the centre for me), Warszawa Praha (not been, wrong side of the river). You can see their current offerings here:
My favourite Czech bar is Vyceska. vyceska.pl/menu/drink-menu-2 If it was in the centre I'd be there every week, but it's in the depths of Ursynow, so I've been there three times in the last year.