Each love interest is linked to an NPC. After you reach maximum hearts with the love interest, talk to the NPC afterwards to unlock the NPC's CG. The chef is linked to Rald, so just reach maximum hearts with Rald and talk to the chef afterwards (at any point in the story - not necessary to do it before the party).
For the other characters:
Garage guy is linked to Oki.
Grindr tanktop guy is linked to Okan.
Bookshop guy is linked to Lenga.
Burton is linked to Remi.
Teacher guy is linked to Sota.
H&M guy is linked to Kovit.
Can't figure out who bandage guy and Dr. Ken are linked to.
All the NPCs are bottoms though. He could have at least made some top/vers.
And i wan to know get page 19 cg