Mukashi Totta Kinezuka de... to Iinagara Musou suru Teishokuya no Ossan, Jitsu wa Densetsu no Dungeon Kouryakusha

A world where dungeons have appeared on Earth. Nishida Kenichi, a 40-year-old man, quit his salaryman job to open a diner, but it was a total failure. After much agonizing, what he came up with was menu development using Dungeon-sourced ingredients & livestreaming his ingredient procurement! He breezes through conquests that would normally be life threatening—just who is this old man!? Viewers, idols, and even S-rank adventurers are obsessed with Nishida's overwhelming strength and mysterious charm!

(Source: Kodansha, translated)